I just got back from an elk trip, and was able to use OnX, Basemap, and Locus Pro side by side in the field. A lot of reviews focus on specific features, but I thought it would be fun here to focus instead on specific JOBS - what you actually use a tool like this to do. Our trip was successful - our group of five hunters took 3 elk - so we hit all the "needs" of apps like this.
"Let's Get Out There!"
I pre-planned several likely target areas ahead of time, using my usual CalTopo.com tools. I downloaded KML files of routes and points of interest (creeks and watering holes, etc.) and imported them into all three apps. I then used them on my scouting hikes. I'm a nerd, but a balanced one. I don't look at my phone every five seconds - more like twice an hour, typically. I use the standard technique of "go that way awhile, following any game trails I find, and check in occasionally to see where I've ended up." Mostly, a mapping app's job here is to record my track and give me a good enough topo to avoid super-bad terrain traps that I could have gotten around if I just went 200yds a different direction.
All three apps worked fine, but for general scouting Locus was a clear winner because it was the easiest to set "guide on" markers to points of interest (with a bearing-to-target mode to keep me relatively on track) and letting me record bedding areas and tracks (with photos attached) I encountered as I went.
"It's 10am. Time for a snack and to see where the heck I am."
Everybody has a different hunt style. Mine is to try to get out to some point of interest like a watering hole, bedding area, etc by sunrise, and stake it out for awhile. By sunset I try to be in a good position near another watering hole or bedding area. During the day, I'm usually poking through dark timber, pausing every 2 hours or so to check my progress and have a snack.
Locus only supports topo while offline, but that's fine. I usually know where I want to be going already, so I'm not doing research in the field. OnX has satellite imagery to help me find clearings and so on, but was a real pain to download. I had to pull one tile at a time to get high-res data and imagery. That said, it was very easy to cross-reference dark-timber to aspen/meadow/clearing transitions and keep headed in useful directions.
Sadly, while Basemap had the easiest "download" feature, its "offline mode" is SUPER clumsy to use in the app. It's hard to figure out what to tap, especially if you have multiple areas downloaded, and it's hard to figure out how to make all the extra stuff it displays in offline mode (like the bottom drawer listing offline areas) go away so you could see the whole map.
I mean, when you start the app offline, literally half the screen is filled with unnecessary stuff, and the default zoom is WAY too wide to be useful. I have three tags for three different zones, and downloaded them all ahead of time. The default zoom shows all three, so it's almost the entire state of Colorado! Honestly, it was so annoying that I stopped using it after day 2.
"I'm 9 miles into a 13-mile loop through dark timber, exhausted, and ready to head back. Get me to camp without too much nasty stuff, but maybe I still want to see a watering hole or two because I might get lucky near sundown"
Here again I really hoped Basemap would be the winner because I had the most high-res imagery downloaded offline. But again, it was so clumsy to use offline that I just gave up. OnX let me pick out a beaver dam and a small pond, and Locus with its guide feature gave me a good enough bearing to wander in the right general direction. Basemap wasn't terrible, it just wasn't "good enough to be worth it."
"Hey guys, Greg took a bull! Here're the coordinates, come help pack out!"
Here Locus was definitely the winner because you can immediately place a pin with precise coordinates, then with one tap, set a guide to it.
Basemap was better than before, on par with OnX. The UI is clumsy: you hit (+) to Add Marker, then tap an easily-missed Edit option in the upper left to enter the exact coordinates. You have to do this in lat/lon fields separately, so if you've been texted a "-117.12345, 46.12345" point you have to hand-type or cut/paste a value at a time. But it works. Once you add the marker, you can navigate to it, but all it really does is show a line on the screen. It doesn't have a compass/bearing mode that I've found.
OnX was similar in functionality, though the procedure was different. You hit Tools, then Add Waypoint, and while you're doing this you can set the exact coordinates (same procedure as above). You can then tap the marker, hit a little [***] menu, and hit Go To Waypoint. But again all this really does is draw a line on your screen. It's not really a "bearing to target" mode. But it's fine.
Wow, remember that awful gully on that last pack-out? That was a PAIN. Think there's any way around it for the future?
After we got home, I was thinking through the last pack-out, which was a pretty brutal, steep descent through thorny brush down into a deep canyon, then an equally steep climb up through heavy fallen timber. We've all seen this, right? Leaning forward til your face touches the slope because if you lean back AT ALL you're going down? And feet sideways because it's that steep?
Basically all three apps were OK here, but with different procedures. Locus has no "web" version to sync with, so you do the steps within the app. You tap a route, hit details, and hit Export. You can share that directly with somebody else, or export the KML/KMZ to your Dropbox. Then pick it up on your computer and import it wherever you want (back to CalTopo for me).
OnX and Basemap were extremely similar. After you're back on your home Wifi, both apps sync with their Web versions, and the Web apps will show your routes. You can export from there.
So what to use?
Overall, OnX is clearly designed by hunters, for hunters. That's a good thing usually, but it has a premium price for a tool a lot of folks probably use a week or two a year. Also, while it's very polished, it's also fairly basic. Simple/obvious features like adding a photo tied to a marker or along a route, seeing detailed stats for a recorded route, or sharing things like current location are less sophisticated than Basemap/Locus.
Locus is the most sophisticated of the apps, and for what it does it's amazing. But it's a general map app, not designed for hunters, so certain things like land ownership boundaries, markers specific to hunting (deer vs. elk icons), and so on just aren't there. Also, for a lot of you, its biggest drawback will be that it's Android-only. I know there are similar apps for iOS, but they aren't quite as sophisticated so may not be enough of a difference that it's not worth the bother.
Basemap is the up and comer. Its much cheaper than OnX, has some nicer features like a better download function, and so on. But it's definitely less polished and clumsier to use, to the point where I actually stopped bothering with it after day 2. I like it, and I hope by next year they refine it further. But those of you still waiting to go out (Seasons 2-4+), I personally would recommend testing it in "Offline Mode" at your house before heading out the door relying on it. You may not love it.
This is all 100% my personal opinions. I'm sharing my experiences, and not specifically recommending any of these three products. I have no affiliation with any of the vendors. I AM a software engineer, so I may be biased by my personal knowledge of how apps should work in general. YMMV.