While many people prefer a classic black and white look for an heirloom portrait, the answer to the question of whether to pick black and white or colour for your portrait is a simple one. It all depends which you like better!
Either way you'll get a print of museum archive quality. And of course you always get to look at a series of proofs before settling on a final choice. The proofs will show a selection of both colour and black and white images so the choice will be that much easier if you're at all undecided.