As someone with a Z3 M Coupe'. There are a couple of "deficiencies" with the Z3s, at least when compared to the Miata.
Weight. Okay, not a super heavy car, but definitely heavier than a Miata.
Along with that, parts are going to be more expensive and will wear faster.
The biggest issue is the rear end. The Z3 was a parts-bin car. It got an E30 rear suspension stuck back there, for the sake of space.
It's a relatively short-wheelbase car, with a semi-trailing arm suspension. That can make transitions in turns a bit tricky, and the back end really likes to step out with trailing-throttle oversteer.
That said, it still is not prone to snap-oversteer. So driving it can be a lot of fun. Especially with big meat on the back wheels. In a turn, you can lift the throttle a bit and put it back in, and the back end will just take a couple of steps out. It's almost like having rear-wheel steering. "I want the back end to step out 1 foot. Okay, another foot. One more. Good."
Because that back end is kind of crammed in there, working on the rear suspension sucks. You have to disassemble the trunk to get to the upper shock mounts. It's a pain. I've seen people, especially ones with adjustable shocks, just cut holes in the padding over the shock towers so they have have regular access to them.
The big killer though, is the rear subframe mounts. They simply are not stout enough. It's just a couple of welds and is really not built as strong as it should have been. There is a well documented problem with the subframe tearing out of the welds. It's well documented though, and can be checked by looking under the padding of the trunk. There are fixes, but they can get costly (I've heard up to $2000, but I've heard people say that's excessive). Although I've also heard of them being done for pretty cheap. If you do beef up the rear subframe mounts, it is supposed to make the car handle much tighter.
You might not have any problems though, especially with a lower power Z3. I haven't had it with mine, but I've also stopped tracking it, in favor of the Miata.
It is something to keep an eye out for, and to be sure to check on a pre-purchase inspection.
Also, the prices on the M Roadster car be pretty low. On par with the E36 M3. M Coupes are holding their value pretty high.