I'd say the biggest factor to mobile detailing is the climate. I'm in Indiana so mobile detailing can be difficult (ie - winter). And correct me if my assumption is wrong, but shop owners have more "billable hours" through a 5-day work week vs. a mobile shop. Being mobile, I realize that I spend a lot of time in my truck driving to/from appointments when a person with a shop is able to spend the entire day working.
The best thing I like about being mobile is the opportunities that come from working in a different place each day. If I'm working in a place where other's can see me, I normally get at least one (sometimes a few) people coming over and asking me for my card and more information.
For the climate I am in, I think a fixed location is definitely the way to go. Although, for me, getting setup to be mobile was easier to start out with.