Here's another thread with basically the same topic if you're interested:
http://www.operationsports.com/forum...rking-you.html
My general rule is guys hitting under .220 or thereabouts, I almost exclusively use the contact swing. They haven't earned the right to try to do anything more. Contact swing also for 0-2 and 1-2, usually.
Normal swing - probably 80% of the time.
Power swing - basically when my literal objective is to hit a HR (3-0 or 3-1 count with runners on, e.g.).