Has anybody sailed Martinique to Antigua or vice versa in April. Trying to get if one direction is best for a one-way charter (either south to north or north to south). Thanks.

Yes, I've done both directions several times. April will get you winds on the beam for both directions and the leeward sides of Martinique, Dominica and Guadeloupe will have you either backwinded if in close to shore or a strong breeze if you go far offshore. I'd choose the cheaper alternative (both for flights and charter) and go with that.
If in late April, I'd end up in Antigua so as to be there for the Antigua Sailing Week.

I'm in St. Lucia right now and will probably head north in a couple of days and do the northerly travel direction.