I have heard both good things and bad about external antenna's...so can't really say on that.
I have heard they are very expensive.
As for the ones you stick on your car...back in the day they worked great for my bag phone, but haven't had to use one since then...so really can't say on that one either.
Myself, I would stick with a tri-mode phone and that would seem to solve all the problems.
I did a quick search on Verizon's site and this is the link to what tri-mode phones they still have available:
Link