If you've already blown through several top ends, then I'd guess the rest of the bike is getting pretty worn too. I'd get a new bike before the 02 starts to nickle and dime you to death. Before long, your bike will probably need a bottom end, clutch and a new barrel. Who knows what clutch basket and tranny look like after two years of racing?
As for the aluminum frame, not having aluminum hasn't hurt anybody at team Yamaha, Kawasaki or Suzuki up until now. Besides being a little lighter, I don't think aluminum is a noticable performance advantage. Does it seem like Bubba, Chad or Davi Millsaps need aluminum frames to be competitive? How bout Honda 125's? They've had aluminum frames for years, but it hasn't helped them in shoot outs.
I don't think you can go wrong with Honda or Yamaha. Go for the one you can get the best deal on.