Power swing is good for the long ball, but the timing window for you swing is smaller and you have to be more precise with the pci. Contact swing is pretty much the opposite. Easier to make contact and put the ball in play, but far less power. Normal swing is the best of both worlds.
Personally, I just about never use power swing. If I'm in a 3-0 count and I need a home run to tie the game or take the lead, then I might. But I'd most likely stick with the normal swing. As for contact, if I have 2 strikes and I need to advance a runner from second with no outs or third with one, then I might use contact swing, but again, I'd probably just stick with normal.
I appreciate your input, as well as your personal preferences, but I'm looking for concrete gameplay differences between the three. For example, if I use Contact Swing, what stats are actually taken into account and to what extent? Will it only look at my Contact R/L stats, and apply 100% of those, while taking only a fraction of my Power R/L stats? I can't seem to find anything in the game that defines exactly what each swing does.
I typically stick to Normal Swing personally, and use Contact Swing when protecting with 2 strikes, but it'd be nice to know exactly what each button actually does.