For me iPaq 5450 + TomTom 3
Wired antenna in the car, bluetooth on the bike. So bluetooth would be the best option for you in this case. TomTom annually update maps etc and it costs around 40 euros to update. There's lots of plugins (like postcodes from
www.didgitools.co.uk )
On the bike stick the pda in a tank bag along with the antenna and it works like a charm. You can plug in an earpiece and get the voice directions as well if needed. I've used it a few times on the bike without any problems. I'm also thinking of getting a RAM mount to use it on a mountain bike too.
Some of the bluetooth recievers are a little "flakey" and have battery issues. Possible the best one at the moment is the Fortuna Clip or it's clones. It's got 2 modes of use and uses Nokia phone batteries whilst not being much bigger than a pack of matches.
This is an excellent site for more info
http://www.pocketgps.co.uk/