The differences are more in the machines and their capabilities than bureaus and operators. I've used a local bureau with a high end Dimension machine because it can produce ABS or ABS-like parts in consistent solid colours with a mold-like semigloss finish. There's a European online bureau called Shapeways that was originally, I've heard, a spin-off of the Philips electronic company. They have a wide range of machines, materials and finishes, their online quote system is transparent, communication is good, and there's lots of feedback from customers about their service and delivery. If you're going local, expect to see examples of the sort of finishes they can deliver, and perhaps the mesh resolution (edge length and normal tolerance) of the .stl to obtain it. Don't discount subtractive RP, CNC milled plastics can deliver surfaces that need minimal wet sanding with flour paper and polishing to look like a molded part and can be cheaper than additive RP.