Answer: It depends!
Some photos just lend themselves to both! A better question is, what are you going to do with the photo? And what's your personal preference? Do you want to use it on Facebook? Make greeting cards? Have a large canvas or metal-print made to hang above your fireplace? A desk-top portrait? A mousepad?
Today, I took this photo of my brother's lovely black dog, Mya. She was sitting in pool of late-afternoon sunlight in our living room, and happily gave me a pretty head-tilt when I made a crazy squeaky-noise. I am really pleased with it - I think its possibly one of the best pet-portraits I've done so far.
There are so many different b/w conversions one can do but, to start with, I thought I'd go low key:
What do you think? Both photos are on my website.