There are advantages to both.
Powder is harder/thicker and resists chips better. However, if an on-the-spot repair is needed, it can be a pain in the rear unless the powder is of good quality and the job was done correctly. As others mentioned, if the powder is crappy, it will gum up when sanding on a spot job. Also, custom colors are iffy since powder clear easily yellows. Blacks are usually easy and come out great and less prone to fisheyes/dirt etc. Matte finishes are great as well and come out very uniform. Powder also builds nicely and would make your surfaces more uniform.
BC/CC is great for custom colors, as clear is usually pristine in clarity. I don't have the greatest luck with shooting matte clear...it usually gives me problems and is difficult to match unless you keep the formula (4 to 1, 6 to 1) or do multiple spray-outs. BC/CC is easily repairable, but not as chip-resistant, especially on wheels that are very close to pebbles, etc. CC seems to handle high gloss as far as glossiness better than gloss powder.
I have shot SPI epoxy in black and it dries to a semi-gloss. I wouldn't personally feel comfy leaving the wheels without a topcoat even though I have shot a set of wheels in black epoxy and left that way for a customer. But as mentioned, scuff, clean very well and shoot.
John