Yeah, the ST-45 is a real powerhouse for only $70. I am flying a .55AX on my Four Star 40 and it is amazing. It has unlimited vertical at 1/2 throttle. I find the OS engines are easier to set up. You just break in per the manual and they are trouble-free. My two Super Tigres have also been great, but they required a little more fiddling after break-in to get them reliable.

If you are not in a hurry, you can keep your eyes out for a sale. Hobby People regularly has some pretty amazing deals on Magnum 2 and 4 strokes. I got my OS .55AX from Tower last December for $119 shipped, and I am not a super saver member.

Good luck