I have Both the Hyper .21 8 port and an OS.RG.
There are a couple of things to consider here....
Firstly the RG is not made anymore and has been replaced by the VG which has a poorer nickel coated liner so probably won't last like an RG which was ABC!

For the comparison.....
The hyper 8 port is much faster in outright speed, and can have reasonable acceloration. It has a 3 needle carb which some people find tricky to tune. I have replaced the 3 needle carb with the OS 20E carb (which is the recommeded replacement) and found little difference and now the 3 needle sit back on it with no problems. Some people report the 0-rings perich on the needles, but a cheap replacement is available if you have problems.

The OS is easier to tune and has better low end power but is not as fast by quite a way on the top end.
Its a lovely begginers and hobby grade engine which makes a nice upgrade from the engines found in most RTRs.
I am wary though of this new liner and probably would steer clear of the new OS engine.

They are both cheap good value engines. The choice will probably come down to whether you want lots of top speed or low end torque and easy tuning (not that the 8 port is hard to tune just not quite so easy).
I hope this helps