I don't know Tower engines, but I know Magnum, TT and Supertigre. My choice would be TT .46 PRO, which was my first choice for my Extra. Fortunatelly, I got almost new ASP.80 FS - I don't regret this at all...

I have a TT.46 PRO on my GP Easy Sport, and it's a hell of a performer. I hauls that 2.6kgs through the air with authority. Verticals are excellent, but after 150m from full-throttle level flight it goes out of breath.

ST .51 would be another one - extremely light, powerfull, but a gas (glow?) guzzler.

If you are thinking about some serious aerobatics, four stroke is the choice. It's beautifull to see your Extra make a 200m diameter looping - slow it goes up, slow it goes down. And about that vertical snaps... When going home, you just can't stop grinning...

Sašo