I own a 330Ci (poor man's M3) and have owned two other 3-series BMWs, so I am strongly predisposed that way for reasons that may or not apply to you.
Basically, Volvo is what it is -- safe, well-built, solid all-round performance, uninspiring design, handling below par if you really want to drive.
The Z3 coupe is a bit of a quirky car, in the judgement of more serious bimmer owners -- it has a following but it lacks the broader appeal that would assure much resale value in the long haul.
The S4 is a hot car with solid performance specs all around. However, in my mind, it lacks an essential element that the BMWs have in buckets -- styling. There is little about the S4 (A4 body) that will turn an eye and I think the styling will appear dated over time. I also think it is a bit imbalanced in its performance specs -- heavy emphasis on horsepower, overly dependent on its Quattro 4-wheel system for handling. It is turbo, so even though it suffers little from tyurbo lag, you are not getting the full benefit of its HP ratings unless you are putting your foot into it. Interior styling is great, seats are only OK.
Now, I obviously am setting all this up to make the case for the 3-series -- the 330Ci in particular. It consistently ranks at the top of all tests of sports coupes. I think the styling is awesome -- growly, confident, graceful. This car is BALANCED -- HP, braking, handling, so you feel more competent that you deserve driving it. The heart of this car is the in-line 6 engine (I prefer the 3.0 immeasurably over the 2.5). It is the smoothest running beast you will find and it, importantly, balances HP and torque through the whole RPM band. Resale beats anyone else in its class. The price seems high for some, but I am a gear head and appreciate where some of the $$ went -- decades of refinement in suspension, engine technology that make me feel confident I will own this car for years. Styling -- it turns heads, especially if you have the courage to get a black one. Shortcomings -- leather is lower quality than it should be, sound system is marginal. Definitely get the sports package, premium package is silly frills. The mirror on the driver side visor is standard so you can lower it and watch the silly grin on your face when you drive it.
Hope that helps.