I owned an RX-8 for 2.5 years, and I currently own a 350Z and a MS6. I spend a lot of time at the track (road courses - did about 10 events this year), and I've never driven a car as spectacular as the RX-8 from a handling perspective. Sure, it is somewhat underpowered, I'll concede that. But, on a road course, it is just magic.
But it's a fickle beast. You have to really love the car to enjoy it, and the harder you drive it the more it rewards you (and the happier the engine seems to be). I miss mine every time I go to the track. The Z is nowhere near the true driver's car that the RX-8 is.
That said, I lap seconds faster in the Z than the RX-8, so that's fun. And, I have a nice MS6 to use as a daily driver. I love the MS6 for that, and it's pretty darned good in its own right from a handling perspective. Much more neutral than I expected. For mountain road blasts, it's quite good. I'm not sure it would stand up at the track without some improvements, but I also don't need it to. As a do-it-all car, the RX-8 was perfect, in my opinion, and the best thing going for the money. But, I really like having the MS6 for daily driving duties and for occasional fun, and having the Z for dedicated track use is nice so I don't have to worry about getting to work if it's in pieces during the week (my RX-8 was track terror and daily driver, so it always had to run - which it did, even with a turbo on it, and proved very reliable and durable despite the issues some have had with them).
So, which one is better? Depends on what you want to do with it. RX-8 wins hands-down if you're looking for a telepathic extension of yourself in a car that gets poor gas mileage, drinks oil, and is somewhat fickle at times but has TONS of character and charm. If you want a great all-around car that has some power potential, a very competent (bordering on great, for a sport sedan) chassis, brake feel that almost matches the RX-8's, and generally a less expensive S4-like sedan (another car I've owned in the past), the MS6 is the pick.
For me, if you need one car to do it all, including going to the track (road courses), it's the RX-8. If you don't plan to go to the track and want something for everything else life throws at you (short of towing something or hauling lumber), the MS6 is pretty darned nice, and with the rebate I got it's just unbeatable.
And, as for the motors, no, they aren't remotely comparable. You spend a lot of time in gear in the RX-8 due to the wide rev range, whereas the MS6 has a small window of usable power between turbo spool-up and the dead zone after 5700-6000. I have two buddies who have RX-8s and did a little speed comparison with one last week. The MS6 is definitely a little faster, but I was much busier rowing the gears. The RX-8 has a silky smooth power delivery, whereas the MS6 is abrupt and somewhat coarse. If you like the kick in the pants that the MS6 turbocharged engine gives, you might be a little underwhelmed by the Renesis, which is the epitome of linear. If you love a flexible engine that is extremely linear and loves to rev, you won't find a better one than the Renesis.