i've had nothing but 2 strokes so far. I'm planning my first 4 stroke on my next project.
If you do decide to go with a 2 stroke, I highly recommend tower hobbies line. They are basically GMS engines, but with schnuerle porting. And of course a purple head.
High on horsies and light on the wallet. I've never had any problems with mine and they're easy to tune.
read this thread too on the tower .75
http://www.rcuniverse.com/forum/m_3457159/tm.htm
Just putting my two cents in