I don't know much about the Tower engines but I would steer clear of the Super Tigers. I've seen several guys down at the field try to get them to tune right and have lots of problems with them. The reason the OS 91 is $100.00 more is because it's clearly a much better engine.
Also the reason that GP warns about overpowering the Do's is that they aren't made for any kind of fast flight or dive bombing and pulling out real quick. You can put a Saito 100 on the 46 with no problems and have lots of fun with it as long as you don't overspeed it too much. Same with the 60. I've got a Saito 125 on my 60 and it flies great but you just don't want to fly them full blast.
One of the guys at our field had a 46 with an AX 55 and he was zipping around the field with it making it try to fly like a P51 Mustang. Well we warned him it was going too fast and sure enough it exploded. There were pieces scattered over 100 ft.