Having been lucky enough to have driven both, it's an interesting decision to make
They are 2 very different cars, a well looked after rotary will give you no more problems than any other well looked after car, I know of 2 people who's rx8's I would buy without a second thought (brett and nathan). All the advice I can give you on buying a 2nd hand rx8, is get a compression test and if the owner/dealer doesn't like you taking it to redline, don't buy it
Being a turbo car, the mps isn't bullet proof either, but you are less likely to have problems from people nannying the engine...
A 6mps is more a grant tourer, and with minor mods, will give a fair few cars a scare on a straight stretch of road.
The rx8 will never go big power unless you put force induction on it, but with near perfect weight distribution, and great manual gearbox, they're more like a bigger less compromised lotus...
You'll probably find a mps more fun to drive on a day to day basis, unless you live in canberra, but for a run through your favourite set of twisty roads, rx8 would be my pick