Gunnerz, The two biggest reasons to purchase a Crossfire over a 350-Z or an Audi, is styling and price. Of course styling is purely subjective, but in my personal opinion the Crossfire wins hands down. The "head turning" ability of the Crossfire (even though it's been around for over 3 years now) is still phenomenal. That can be a good thing, or bad, depends completely on the individual.
Initial price of purchase, is as we all know a steal for the Crossfire, and way better than either the Z or the Audi.
When it comes to comparing engines, the Z is nearly 3/4's of a second faster in the 1/4 mile than a limited Crossfire, the Audi is approximately the same as the XF for that distance.
The Z while being more powerful, is not as smooth as the XF, and the XF motor is quieter under hard acceleration. The Audi's Turbo Charged 4 cyl. is of course nothing more than a 1.8L Volkswagen engine. (insert your own opinion here).
Handling is very similar between all three car, but from road tests I've seen, the Crossfire comes out on top thru the slalom.
On the negative side (according to most sports car buyers) the Audi is front wheel drive, but can be had with all wheel drive for way more money.
Resale according to Kelly Blue Book.com, for the 44260 ZIP Code on these cars with 15,000 mi. and automatic transmissions is:

Suggested Retail Value

$25,640 ... '05 350-Z Enthusiast Coupe
$31,750 ... '05 Audi TT Coupe
$25,375 ... '05 Crossfire Limited Coupe

Once again, we all know these prices are high, since a "New" SRT-6 can be had for about that same amount. But that's how the Blue Book rates them.
For everything the Crossfire has to offer, and the incredibly low price they can be purchased for, they should be selling like "Hot Cakes". I think it's really only a matter of time before "true" sports car enthusiast pick up on that fact and these deals start disappearing.