That is a pretty significant price point difference per year. Agree with the posters above that the M3 and Z4 are completely different cars.
But with the money you save being its a 3rd car, I honestly think the Z4 might be the better option. As a third car the Z4 is pretty sick and I would bet you might put the top down on more than a few occasions. Open air in the cool evenings makes for some great driving enjoyment. For 325 a month less than the M3, that is a pretty good deal and the Z4 is gorgeous.
That being said, the M3 is wicked fun too but in a completely different way. It sounds like to me that you want to be frugal being its a third car. As a third car, I think the Z4 may better fit what you are looking for. M3 is a damn good daily driver and weekend track car. It doesn't sound like you will be doing either of those with the car so I suggest the Z4. And who knows, you may love dropping the top.
08 GT500 convertible red package (Black w/ Red stripes) /Ford Racing Exhaust /Cold Air Induction 560hp
10 Porsche Cayenne GTS Black w/ Black Wheels 408hp
11 M3 Silverstone w/ Silver Novillo Leather /Tech /Premium /19" /DCT /Ipod 414hp