ORIGINAL: bigedmustafa
The one thing not yet mentioned in this thread is the weight of the engine. The Tower Trainer .40 Mk II ARF balances most easily with an engine in the 12 to 13 ounce range.
The Tower Hobbies .46 engine weighs in at just under 17 ounces and is too heavy to allow for balancing the plane properly without adding a bunch of weight to the tail. I don't know why Tower Hobbies uses this engine for their RTF package, but their own owner's manual shows where to put a big strip of extra weight onto the tail where it talks about balancing the CG (center of gravity).
The O.S. Max .46 LA, aside from being very user friendly and providing plenty of power for the Tower Trainer .40 Mk II ARF, also happens to weigh about 12.5 ounces with muffler, which is exactly the right weight for your airframe.
Is setting the low end on an air bleed carburator difficult? Maybe. If you buy a .46 LA for your trainer, however, odds are you can fly the thing for five years and not have to adjust the air bleed screw. We have students in our club who've been flying their Avistar and Superstar RTF trainers for years who don't even know there is a low speed adjustment on their O.S. Max LA series engines.
The .46 LA isn't the only perfect fit for your Tower Trainer .40 Mk II ARF. You would also do well to consider the Thunder Tiger GP-42, which is a bit more powerful and perhaps the easiest engine on the planet to tune.
If you would prefer a ball bearing 2-stroke, there are plenty of very nice choices in the .34 to .36 displacement range from Magnum, Super Tigre, O.S. Max, and Evolution that offer plenty of power for the Tower Trainer .40 Mk II and also fall into the ideal 12 to 13 ounce weight range.
A lot of people will tell you to just slap a .46 ball bearing 2-stroke on the nose and don't sweat the extra weight. The problem with that is you'd be adding an extra 12% or so to your wing loading and your plane's gliding and landing characteristics would be adversely affected. Your trainer would feel heavier in the air and would land faster carrying the extra weight.
Fly the .46 LA or GP-42 with prop in the 11x5 size range, or a ball-bearing .34~.36 engine with a 10x5 prop, and you'll be rewarded with a trainer with plenty of power that isn't nose heavy and has the proper glide and landing characteristics that a trainer should have.
Good luck and good shopping!
I forgot to add that there is a trainer ARF that is the perfect match for the Tower Hobbies .46 ball bearing engine. Ironically enough, it's the Tower Hobbies Tower Trainer .60 Mk II ARF. The Tower Hobbies .46 will provide plenty of power for the bigger trainer and allow it to balance easily as well.