If your serious about your windsurfing then you’ll care a lot about the kit you will have in Maui.
Rentals
For intermediates who plan to improve a lot, I think hiring is an obvious choice – you can change board every day if you like, and work your way down from bigger boards to wave kit if needed. For short stays, with no time to amortize excess baggage costs, and when you want to be on the water almost every day regardless of the conditions, it makes sense as well. You should be able to get 30 days rental (1 board, 2 sails) for about $800 for 30 days, which beats buying kit, and comes close to bringing your own.
Bringing your own kit
If you already have gear you’re comfortable with, and you are staying long enough that excess baggage fees don’t worry you then bringing gear with you is a good option. However it’s worth considering that the reason Maui is so attractive is probably that the conditions are different to home! So your normal gear might not be what you want to sail on Maui – a combination of bringing gear and renting by the day when conditions don’t suit can work well.
Buying in Maui
If you’re happy to buy second hand kit, Maui is paradise. There is a huge supply of gear, from ex-rental gear, local pros, photo shoots and the local sailors. It shoudn’t be hard to turn up with nothing and have a full complement of kit in a day or so. One big advantage is that the second hand gear for sale is going to be matched to the local conditions. There’s also such a glut of gear that there are some great deals available. If you’re staying more than a month it starts to get cheaper than renting, and if you go for a mix of bringing gear and renting even more so. e.g. if you bring some sails and buy a board.