If you just trail ride and plan on keeping it for an extended time then the 4 stroke is a great choice. The 4 strokes lifespan is much longer than a two stroke. Also no oil to carry , no smell , quieter, and better resale. I don't think you would be happy with the 600 ace though. After you get use to your sled you won't be happy with the power of the ace on long trail especially when you cross a lake or on a fire road. Most trails don't stay in the tight and twisties. The 900 would be a better choice. Check out Doo Talk and look at the 4 stroke forum and you will find tons of info on all doos 4 strokes. I switched to a 1200 4 tec from a 600 2 stroke and I love my 1200 renegade x. It is hands down the best trail sled I have ever ridden. Good luck with your choices and for sure take a look at the 900.