I have the Z4 roadster and love it. I find it quite spacious inside (5'11 avg weight) and has the ability to carry a full cart of groceries AND still maintain top down functionality. I have also fit 444 cans of beer in my z4 in case you ever need to know that.
The other reason I went with the BMS z4 is because of the 100k mile service warranty and 50k parts/labor warranty. So the cost to own my z4 is actually a bit less than owning my GTI. You can even get a low mileage M roadster now for a very good deal. I would look into off lease CPO's. It uses the old 3.0L engine, the same as found in other 3 series which means it's been around for awhile and is super reliable. I have over 65k on mine with nothing but oil changes in it so far.
I would stay away from Porsche, they're $$$ to maintain and hard to do the simple stuff (oil changes) on your own.
Lurk on Zpost and z4 forum, you may find someone selling a good one on those sites for cheaper than what you'd find on cars.com.
Also look at completed auctions on ebaymotors to get a good idea on what the cars are actually selling for.