You'll never go wrong with OS. Pretty near 20 years experience tells me that in the long run you do get what you pay for. Quality engines like OS, Enya, Webra and Saito tend to require less fussing with and provide better long term reliability. If you're not particularly worried about those qualities then go ahead and buy a Thunder Tiger, ASP or Magnum.

My Four Star 60 is powered with an Enya 90 FS. It is a wonderful combination - power, grace and even a bit of attitude. My brother used a pumped OS 61SF on his but he also clipped 2 wing bays off each panel so it would be hard to make a direct comparision between 2 and 4 stroke flight performance, but I will say this, much less needle tweeking with a 4 stroke. Once they're set they're pretty much set.