A friend of my sisters tossed a separate money clip with $50 or so and ran (spare wallet sounds good to). You may want to plan out where you'll keep it though, just in case they see you put the real one away (e.g., tracking you at a store).
If they haven't shown a weapon yet, maybe ask to see it? get them talking, offer a compromise with the intent of getting some distance? oh yeah, grabbing their weapon? might not be good to rely on anything with moving parts.
Probably still a good idea to keep your eyes on the mugger as they (or you) leave, I've read elsewhere of someone that handed over their wallet, turned and walked away (as instructed) and was shot in the back.
Driving around with someone doesn't sound good to me, too many things you're not in control of at that point, might be constricted by sitting down w/ a seatbelt. seems like the same situation, except you're in a box moving at speed. If you shoot him/her, you have your bullets to account for, and the other projectile: 3500lb car at 50fps? (dodge magnum might be appropriately named here).