I've sailed solo, both with a singles group and totally on my own. I had a wonderful time on each of those sailings. I highly recommend going the singles group cruise route. At least you know that way you won't be the only single on board. You're guaranteed to meet other singles. You run a risk of getting stuck with "the psycho roommate from hell" if you choose to let the group find you a roommate. I was lucky on my last cruise to be matched up with a very nice roommate, but I've seen and heard stories where things didn't turn out well with others. If you do the roommate matching thing, try to get in touch with your roommate prior to the trip. That way you won't be total strangers from day 1.
If you don't mind going it alone, without a group, you may end up with a hefty singles supplement added to your fare. When I went on my own I found a 'too good to be true' singles rate at the last minute. Well, it was worth every penny! I thoroughly enjoyed having the cabin to myself - no bathroom times to schedule, no worries about if your roommate will snore, etc., etc.
My next cruise is the singlescruise.com Canada cruise 7/2-7. I'll be going with a friend who I've traveled with many times, so I know I'll have a good roommate.
Whatever you choose will be great - cruising is the way to go!