Definitely get the Porsche Boxster. You will learn that speed isn't everything when it comes to driving enjoyment. The way it steers, the way the car talks to you when you turn in, and the general tightness and precision of it is something else. Before I bought my Boxster, I tried an SLK AMG - you can really feel all the weight down the front and its reluctance to take the corner. Because the front engined car felt so unbalanced, lift off oversteer was a problem as well. I don't mind a little bit of lift off oversteer, but the Merc was a little too much. If you are not anticipating it, it will punish you badly. No such shenanigans with the Boxster.
By the way, I happen to think that the Porsche Cayman is the best sports car, at any price. Reasons - it is mid engined, it handles beautifully (even better than the Boxster), it is comfortable, it actually has enough space to put stuff in, and you can drive it every day. There are cars that are faster - but in reality you won't need that kind of speed unless you do a lot of track time (in which case you are better off with something more specialized). The Cayman, and even slower cars have enough speed to make you lose your license very quickly.
I prefer the Cayman to the 911 simply because I don't like the way the 911 handles ... you always get the feeling that the 911 is trying to kill you. What about other mid-engined cars? Ferraris and Lamborghinis can not be driven every day, and Lotus cars are not comfortable or practical. The Toyota MR2 is no longer in production. What about front engined cars like the M3 and the Audi TT RS? Those cars handle like front engined cars. The first time you turn the steering wheel on a Cayman in anger, you will know what I mean. The change of direction is instant. The steering wheel loads up just nicely and you know exactly what the car is doing.
My suggestion - either a Cayman or a Boxster. But IMO the Boxster is more fun. Ultimately, driving should be about pleasure and not necessarily about speed.