Z is the speed rating and has nothing to do with the softness of the rubber compound.

As far as the actual question I'd say you have two options. You said you want to go with 215/45-17 right? Readily available online in that size you have the Bridgestone Potenza S-03 Pole Position for $146 each (plus s&h) from tirerack.com or the Yokohama AVS Sport for $159 each with free shipping from discounttiredirect.com. The Bridgestones will give you a bit longer tread life than the Yokos but the Yokos have a tad more dry grip. In the rain they're dead even.