I just had to go through the same decision earlier this week
I ended up opting for the base + sports. There are a few things I miss from the touring/nav, but not enough for me to be able to justify the premium without any buyers remorse.
I just feel like I got much more value for my money with my Z, then I would of otherwise. Also this economy has got me a bit spooked so I figure shaving some cash off now is never a bad idea!
Only thing I am missing currently (after 24 hours of ownership) is the sound system. But hopefully it won't be too hard to upgrade that in the future.