That would depend somewhat on style of voice and music, but overall, the "A" would be the one. A situation where off axis rejection wouldn't be a factor the "C" would be a better choice for someone who likes to play away from the mic.
The "A" is a more common mic on stage. In fact the Beta87A is probably the most common condenser vocal mic on stages these days, not just between the two.
Have you compared the Beta87 with the Beta58? The Dynamic Beta58 is the more useful "live" mic in most situations. I would recommend an A/B comparison between these two before purchasing either one.
If this is for "Live use"... Why in the "Recording" forum?