First off, Welcome to NICO!
That's a lot of miles on that 370Z, especially considering how relatively new it is. The good news is it that it might be a lot like my '05 and had its miles piled because of a long highway commute. They are nicer on the interior, both in layout/design as well as in the quality of materials & construction..that funky gas gauge and pointless clock notwithstanding.
On the other hand that 350Z could be a great find, low miles could be a car that was babied, driven easily and we'll taken care of. It could also be a car that had chronic problems and consequently sat around in a garage or shop a lot.
I like the way the RX looks, but the rotary isn't exactly my thing unless it's for toy car. Poor fuel economy, oil consumption, low torque rating and occasionally even difficulty to find a quality mechanic van make ownership a little stressful. I leave it out of any further questions & evaluations because I can't really advise you on it much beyond what I've already said. It is pretty telling that not even Mazda uses the rotary any longer.
A couple of questions...
What are you looking for in the purchase, i.e. What's the intended purpose/use?
What are the trim lines of both the Zs? Are the similarly equipped?
Have you had a ppi (pre-purchase inspection) performed on either either of them?