This is on my mind, too, as I'm in the initial stages of planning my own location.
My general feeling is that transport links are most important in terms of clients. I'm in a small town, but near a biggish city, and more to the point, only a five-minute drive from a junction of the M25. So I get both locals shuttling around the area and commuters who are driving round the M25. I'm also within a 15-minute walk of a tube/train station, and I get some visitors from London that way.
I think smaller towns are fine if the road links are good - a lot of drivers avoid city centres. On the other hand, cities are good for men who will walk or get a bus, train or cab. They're also a bit more anonymous.
If this is just going to be a working flat and you're not going to live there, I would prioritise a). being close to a major road, b). free on-street parking nearby, and c). both the flat and the area being generally 'nice'.