This question does not have a right answer. It's like asking if you should drink Coke or Pepsi, it's completely subjective.
Every domainer has a different strategy and each strategy - in most cases - has its own merits.
Personally, I buy domains when they are undervalued. I don't care about the niche at all. My strategy mirrors my personality, in that I like to skip, jump and hop from one thing to another. I've never been seriously interested in a single subject so buying domains within one vertical does not interest me.
However, if you have a wealth of knowledge on a single subject you may be better equipped to buy meaningful domains within that vertical.
So pick the strategy that interests you most..