As of right now, both of these motors run very well and are very competitive on a small track where a lot of horsepower is not required.
The RB comes with a small head that does not use a button. You will need a larger head if you can get it.
The Nova is the one to go with on a budget.
If you are tight with your money go with the RG by OS. It will make the same power as the other two, is easy to tune and you can pick them up for $100. For an extra $25 (over what you paid for the other 3 ports) you get two motors.
I race the RG on a 175 X 70 track and it does not get out motored.
Hope this helps,