I've found that even though O.S. is a very good engine they tend to be a bit more pricey then the rest. Thunder Tiger is a knock off of the O.S. engine but the price is a little better (Tower Hobbies has an O.S. 46 AX for $110 and the TT Pro for $88). I don't believe the quality is any less either IMHO. There are other good engines as well. I've owned the same Supre Tiger 45 for 15 years and never had a single bit of trouble with it until I buried it into the ground just recently (Someday they'll make an engine that will survive a crash from 100+ ft ) Super Tigres tend to be on the bulky side though. All these of course are 2 strokes and they are ABC (Aluminum, Brass, Chrome) construction as apposed to a piston with rings (ABC construction gives you more power, but when they wear out you are unable to rebuild them as easily as a piston and ring set up) If you are interested in 4 strokes then that would be allot more money but gives you a very cool sound. I would recommend you stay with a 2 stroke.