Difference between a professional bike restoration place respraying and power coat company?
A really good question and one that a pro in the bike painting/PC business would be best suited to answer, but this is what I know about it:


Bikes need special care, or at least an operator that understands the important parameters, in the stripping, blasting/prep, and heating stages. For example, I've heard horror stories about taking a frame in to a general industrial powder coater or painter and they used a torch to assist in getting the original paint loose, but got overzealous (i.e. didn't calibrate their actions for how little mass bike frame tubes have compared to their typical work) and destroyed some brazed joints and braze-ons. Similarly, a lot of blasting media is well capable of damaging brazed joints if not used with care. Bike people have equipment setups and technique that make it a non-issue, but bad things sometime happen when more general purpose companies do it. (Note that I'm not trying to pigeonhole anyone, and there are a lot of very thoughtful people that work at said general purpose companies who can correctly handle any metal object you give them, but I do think the risk is there.)
Finish work on bike frames well is more highly specialized than many imagine, especially when it comes to showcasing and not obscuring fine details, particularly on lugged bikes, and especially on upper end/handmade ones where the builder has done lots of thinning and shoreline work. With powder in particular, companies are mostly in the business of putting big thick economical coatings on relatively smooth, round, heavy things. There's a much smaller subset of powder companies that specialize in thin but durable coatings like what you'd want on bikes. (There's also a whole other discussion about how thinner might be inherently more durable with various finishes, especially powder, at the trade-off of taking more work to do right.)