I am building a sonex. The sonex and RV are very different aircraft suited to different roles and budgets. The sonex is much smaller than the RV with far less endurance/range. The sonex will be much cheaper to build. Performance depends on your choice of engine. You can choose from the VW (110kt cruise from talking with owners) up to the 3300 with owners claiming between 120-130kts IAS in cruise. The big thing to consider is that the sonex only has around 65L of fuel, so if you are burning 20-25L/hr with the 3300 it wont keep you up as long as the RV.
If you want to fly long distance with a passenger and baggage than I might reconsider the sonex. My ideal flight is around 2 hrs, no baggage, occasional passenger and performance like a little fighter.