I had two Fastfire 3s go bad on me..Exact same problem both times. The sight settings would go whacky when I turned it on and eventually the dot would just cut out..Both times I was in the middle of a hunt and each sight lasted right at a season and a half..Cost me a gobbler the first time, but I carried a tool with me to strip it off the second time and ended up killing the gobbler with the rifled sights on my gun. Burris did honor the warranty both times. When my third sight came in, I sold it and bought a Venom.
I've been using a Venom for two seasons now with no issues. I even got to test the 'no questions asked' Vortex warranty this spring. I slipped in a creek and landed right on my sight.
Sent it in to Vortex and told them exactly what happened. Had a brand new sight at my door in under 3 weeks! Can't beat that..To be honest, I wouldn't have even been mad if I didn't get a new sight. It was completely my fault. And the red dot still actually worked, I was just afraid to use it.
As far as which sight do I like the best? They are really about equal to me. But i've read about a lot more problems with the Fastfire 3s than the Venoms. So I'd stick with a Venom.