Brian, I agree with you 100%. Most of the UAS newby's want to take a 50 HP engine and want to make it faster and that is the wrong approach. If they would concentrate on learning to drive the extra HP, making the engine package as light weight as possible (within reason and using some logic) and last but not least, put a fuel delivery system on the engine that will allow the engine to follow the throttle and be 'crisp' when you press the peddle down, they would have an instantly competitive kart that would run for a bunch of races without problems. The main objective to having a bunch of extra HP is getting it to the ground.........
Now, in all fairness to Jimbo, the bullring track is the great equalizer. You can have the 500 HP and go from first to last in an instant, however if you can get it hooked up, you can go from last to first in that same instant.
Billy, pick the powerplant you feel the most comfortable with, can get the most support with and the type of tracks you will be mostly running on. As far as longevity and expense, the MX probably has the advantage but it will also be the most difficult to mount on the kart (compared to the Animal). Just pick one and go racing.
Hope this helps you a little and sorry for temporarily hi-jacking your thread....