Brightkite seems to have the most buzz right now with its integration with mobile Twitter. It is designed to be compatible with most cellphones, even older ones. But in order to do that, it somehow relies on built in text messaging, which for me means it could impact my iPhone's AT&T account allowance limit on SMS messages.
Loopt is more powerful in functionality, but it only runs on newer smartphones and there isn't any recent buzz about it.
The ideal solution would be for the highly popular Facebook to build and release their own location specific feature, but who knows when that will be available.
Some people believe Facebook has plateaued in its popularity, just like MySpace and Friendster before, so there is a scramble to identify and crown the future's next dominant social network. Many believe location specific functionality, if configured usefully and
competitively, will be the determining differentiator. I am not entirely sold on this idea, but I also missed the importance of Google and MySpace as each rose in popularity.
I guess I will keep trying both Brightkite and Loopt until it becomes clear which takes the lead.