Terry, here's the specs:

OS 55AX:
Practical RPM: 2,000-17,000
Power Output: 1.75 ps/16,000 RPM
Weight: 14.25oz (404g)
Recommended Props: 12x7-8, 13x6-7


Tower 75:
Practical RPM Range: 2,000 - 16,500 RPM
Weight: 652g (23.0 oz) (with Muffler)
Horsepower: 2.2 BHP at 15,600 RPM
Recommended Props: Break In: 12x6 or 13x6

So what I'm seeing here is that the OS is 200+ grams LIGHTER and will turn more RPM's. It also calls for a higher pitched prop. The 75 has a bit more HP, but does the tradeoff in size and weight negate that? Also, the physical dimensions of the 75 is significantly larger...so you have more drag from the bigger motor.

Me thinks it's time for a RACE!
I'll put my money on the OS, I think.

OK, time to get away from the PC for a while! It's a BEAUTIFUL day here in Boynton Beach, FL. Check out the beach:
http://www.co.palm-beach.fl.us/webcams/slwi/dig001.jpg

No waves today, can't surf, but I think I'll still go and get wet. I live about 5 minutes away. Lucky me!