Alright since you're going to be using it in your car, the iPod is your better bet since there are literally TONS of car adapters for it. Even if you buy a new car stereo, it will most likely be "iPod compatible."
The only problem is that you have WMA's. Which format do you have the most?
Currently, the iPod comes in 80GB ($249) and 160GB ($349). While the Zune only comes in 4 ($129), 8 ($179) and 80GB ($249). The FM tuner does not cost extra.
They both support the same video formats (H.264, MPG4), except the Zune also supports WMV's. They also have equal screen resolutions, but the Zune is widescreen.
The Zune has better audio out from the headphone jack, but you won't tell the difference unless you use good headphones. So if you plan on buying nicer headphones down the line, then the Zune is your choice.
Both are supported by the Xbox 360 and PS3.
The iTunes store has a lot more content, but most of it is copy-protected. Whereas, the Zune store is DRM free. The iTunes store does carry DRM free content but not as much.
The Zune has WiFi capabilities so you can transfer media wirelessly.
Both 80GB models claim 30 hours of battery life (the 160GB iPod claims to be 40). The iPod claims to have 5 hours of video playback whereas the Zune is only 4. It also depends on what formats you're using. The bigger the files, the more battery you use.
On paper the Zune is better, but if you want a bigger HD and a crapload of accessories then the iPod is what you want. I believe that is it. Any more questions, just ask.
PS: if you don't mind hacking whichever one you get, then pretty much all the differences go away. It does void your warranty though and there is a chance of bricking it.