If you're willing to build, I would suggest the U.S. Aircore trainer. It is havier than a LT-40, which means it's a little harder to fly (but easy enough), and it has the advantage that it's almost impossible to damage it or the engine in a crash.

On the issue of the O.S. .46 FX vs the Thunder Tiger: I have two O.S. and one TT. At first I thought I prefered the OS because parts are more easily available (via Tower Hobbies), but now I realize you NEED parts more! The TT has proved to be much more rugged in crashes. In particular, the O.S. needle valve will break off ($9 part) if you look at it sideways. Personally, I don't think the slight safety advantage is worth the hassle of the remote needle valve, but then, I do all my adjustments from behind the engine. In terms of power, starting, idle, and reliability, I have found the engines to be almost identical over the two years I've owned them. If anything, my TT is more powerful, but it could be the muffler or the tuning, and I do almost all my flying at 1/2 throttle or less anyway.