I have owned a straight and angled spotting scope. I personally prefer the angled spotting scope now that I am comfortable finding my target (this took some time getting used to). The only pitfall to the angled scope is that when I let someone else borrow it to verify their findings, they can't find their target very easily.
As for using it from a vehicle, I prefer the angled. When using the straight from the vehicle I always had to kink my neck to get a good view through it. As for the angled, the spotter I own is on a collar like BB mentioned and I rotate it when in use on a window mount. The result is almost a 180 degree view by only rotating the eyepiece instead of having to adjust the angle of the vehicle.
The angled spotter I have can be rotated 360 degrees thus the angle of the eyepiece can even be angled downward hence making it easier to use at extreme downgrade angles. Angled is the way to go if you use your spotter often and spend hours looking through it. On the other hand, if all you use your spotter for is for verifying a distant object or needing a closer look at that buck once in a while, then I would go with the straight spotter.