The Rx-7 is faster because it has a twin turbo system and makes more torque, plus it weighs about 300 lbs less. A big negative on the RX-7 is even if you buy a used one it will run you $15,000 to $25,000 for a decent one and they will most likely be highly modified. Another negative is even if you get a completely untouched stock RX-7 the Apex Seals in the motor don't last forever and the engine may need a rebuild shortly after you get it. Typically they are rebuilt with about 60-85k miles on them. It is a great car but unless you are someone that likes to work on your car often you will spend some money on it.
On the other hand the RX-8 can be bought new for almost what a used RX-7 would cost. I bought my 6 speed RX-8 for $26k it had a $5k discount because it was last years model, although it was still brand new. I have a 100k mile engine warranty and a 5 year bumper to bumper. It also has a 5 star safety rating with 8 air bags and a traction control system that works great. If you would like to, you can do some upgrades on the RX-8 which does improve the power. I added just a few upgrades and added about 30 rear wheel HP which in a car that weighs only 3,000 lbs is pretty noticable. My RX-8 runs the same quarter miler as the stock twin turbo rx-7 does now (13.85). I added a cat back exhaust, upgraded ignition system, and also a K&N replacement filter. The RX-8 also has back seats with suicide doors which the US version of the RX-7 does not have back seats.
The RX-7 is a collectors car these days and it is nice, but will require more maintenance.
If you want to make big power and have a toy get the RX-7, you can easily make 400-600 rwhp with it. If you want a very nice decentley quick warrantied sports car get the RX-8. They both handle better than many sports cars.