Webwork - 11:16 pm on Jan 13, 2013 (gmt 0)
My answer depends, in part, on the answer to the question "Why build A or B?". To increase, "show" or test the value (for SEO/other purposes) of the domain? To build a website just because you have think you "the domain" that will support the project?
Also, what's the competition in either space?
What's your knowledge of either A or B?
What resources (time, money, other) you are prepared to devote to A vs B?