First a bit of terminology, Live Sound Company = PA Company = Sound Reinforcement Company.
All the same.

If I were choosing a PA company I would look for the following:

1)	A company that does music work - There are companies who do primarily music concerts, and companies that do primarily corporate work (i.e. audio-visual for meetings, conventions, etc.). As a future client who called me this summer, you may call a corporate type sound company and they would give you a quote at a (typically much higher) corporate rate. So not only do you get someone with debatable music knowledge, but you pay considerably more.
2)	If you're recording gospel , find a company that has done gospel work - Gospel brings with it certain challenges, not the least of which is large groups including choirs, church acoustics, and sometimes very little AC power. Better to go down that road with someone who's been there before.
3)	A company that is familiar with your particular venue - As above, someone who knows the quirks & surprises is good to have on your side
4)	A company you feel comfortable with - Completely non-technical, but how you feel after talking with a vendor is every bit as valid as the technical specification involved. If you don't feel right sending them a deposit and trusting they'll show up, move on baby.
5)	A company that has suitable equipment - A system should be large enough to handle the room. A site survey is ideal, but if they don't at least ask how many seats, etc I'd be a little suspicious. I'll be happy to field technical questions if I can help.

Goin' Mobile Mobile Recording Truck (617) 232-7969