I've stayed at both. Ocean Blue/Sand in 2007 and IB Punta Cana in 2003. Either of them would be a great choice, but I would tend towards Ocean Blue/Sand.
The only thing I liked better about IB was that the landscaping is more mature and gives a more lush feel. OB/S has far more choices of restaurants and bars and offers more selection in premium/international brands of liquor. Mikes coffee shop (somewhat like a Starbucks) at OB/S is worth going for. Also, unless they've changed it in the last year, you don't have to fuss around with reservations for the restaurants. Personally, when I'm on holiday I don't want any deadlines (get enough of those at work). I prefer to go down to eat when I feel like it, and not have to rush to get ready, or wait around for my reservation time when I feel like eating now.
A downside at OBS is the timeshare vendors. They can be annoying, but I found after the first couple of days of saying "no gracias", they tended to recognize us and didn't bother asking any more.
If you have any other specific questions, feel free to ask.
