Have used a Viper HS LR 4x16x44. It has been great and tracks perfectly so far. If I bought another, it would just a HS in the same size. I've had the elevation knob move when using a scabbard on a horse. I waited for a Cabela's sale and got it for about 350 or maybe 400, forget, put twas a great price.
Relative to the ffp comment above, I think that may not be the whole story. As I understand things, it is not the bullet poi, point of impact, that changes in a second focal plane scope, but the hash marks relative yardage zero, so they usually just tell you at this X this hash = what ever. But the center x should still retain the same yardage zero if power is changed. Hopefully someone knows. I have all sfp scopes, mess with power and only use the center x, without problems so far.