The motherboard won't improve chances, and there aren't accurate numbers out as to what the success rate is. Rumors are that it's around 70% based on forum results, but people speculate it's actually lower because of people claiming theirs is stable when it's not.
If you need a quad, buy a quad, don't bother with this gamble unless you can settle for the dual core your purchasing.
I would say the chances are equal for both. It has more to do with the CPu than the motherboard. Both of those motherboards can unlock the cores but the quality and stability is the question. For just as many reviews where the cores were unlocked, there are also many where the cores were unlocked but afterwards the system became unstable or the CPU ran too hot. Nothing such as a free lunch.