Hiya,
we are going to be there then!! (well 6th to 17th)
When we researched, we asked around - Last time we picked hurricane season!!! big mistake, hated the weather/heat/humidity so the consensus was springtime for local conditions to suit us (kids at school so our choices are limited!)
Virgin Rep said local spring break is coming to a close around then, so the weekend (5th/6th)will be very busy, but after that it will probably be "mainly the Brit contingent and therefore a bit busy, but nothing like the 4 weeks prior". A call to some villa agents also said pretty much the same thing - and that the second week should be quieter(12th ish onwards). A friend who was living out there 2 years ago said the weeks to avoid in spring are the last 2 weeks of March - but it can be very busy the first week in April too.
So, I guess the later the better - if you have the luxury of choice!
Cheers,
Tessa