Yes serial GPS is cheaper. It may well be much more versatile as well considering you can attach to any computer you can get in your car. On the other hand it is not so easy to use when you are walking around outside of your car. And it has cable clutter. I really had a hard time chosing between serial and CF. See one here;
http://www.deluo.com/Merchant2/merch...tegory_Code=GR