I've chartered in June several times and have always had great weather. For my crew its more about the vacation and less about the actual sailing so we don't mind if the winds get light, we just motor-sail. We like going in June/July when the crowds are lighter, the pricing is lower and the land establishments are still open. So to answer your question directly, given the choice between April or June, we'd suggest June. However you can't go wrong with either choice!