well the first thing I would do is an internet search--surely people have posted something about their experiences. You can ask if they do Trivalent or Hexavalent chrome. Hex is nasty stuff and heavily restricted, Tri is a 'little' more environmentally friendly.
Hex can produce a better look that we're all after, when done right.
Given the choice, I would choose a hex shop.

You expect them to plate a heavy amount of copper, then level it (sand/buff) thats how they remove surface imperfections. Ask.

Ask what level of 'straightness' you can expect. Thats pretty subjective though.

You want a shop that knows you are expecting "show-quality" --again subjective, but at LEAST make sure they talk the talk.

Some people are concerned about the backside of bumpers--do they do ANY smoothing of the back surface? If YOU care, ask.