Starting a Server...

It is incredibly difficult to make successful DarkRP servers because if you have no players you will get no players, and there are so many other empty servers that can't get populated that your chances are almost nil.
PrisonRP would definitely be easier to populate as long as you can manage to get about 10 other friends on to get it started. It definitely needs to be a keeper as well, for which I would suggest having something to earn/work towards that would attract players to come back. For DarkRP that's simply earning money with printers, yet for a PrisonRP you could have mining with an upgradable pickaxe to earn money.