Which engine design is more reliable with less recalls or problems?

I will be riding it mainly on the dunes. I weigh 170 lbs and don't ride with a passenger. Will the 500 have enough power to climb the dunes?

 

500 will have more than enough power for climbing. 800 uses a lot more gas. At 170 lb, you'd probably have more fun with a slightly lighter (smaller) machine. Both are very reliable if you are planning on a new machine. The 500 has a pull starter backup for extra reliability (not real happy with small stock batteries).