If you are contracting sound, I would start with 58s if you do not have them already, as they are the most commonly requested mic on riders (at all levels) and very rarely the "wrong" choice. If an act is specifically asking for an 87, they will likely request either the A or C, the one requested being the one that you do not have
You could always subrent the 87s or another mic from a bigger shop if they are really needed and pass the cost on to the promoter/band/whoever. I would also look into the rest of my system before spending $250.00 or so each on vocal mics. Would you be better served spending the money on a decent 2 or 4 channel comp? Do you have good backline mics/DIs? Is your mixer up to par? Do you have a good multipinned I/O for faster more efficient setup and teardown, so on and so forth?
Myself, I own 4 87A's and 4 87C's. They come out only:
when specifically requested
when I have the time to try both and/or another mic to see what works
To throw another possible solution out there, if you are not specifically trying to meet riders but want to offer a vocal condensor, try the Audix VX5. It runs about the same price as the 87, does not have the 8k-10k harshness that I find the 87 has on many vocals, and has a tighter pattern than either 87 so it picks up less stage wash.
Last possibilty, and probably the best, is demoing/ renting each of the candidates to see what works for you.
Good luck.