Certainly the Boat(Brand, Model, Age) & Cost are important but the Customer Service counts too especially on your 1st charter. The big 3 (same company) Sunsail, Moorings & Footloose will present one style of chartering. The smaller companies will give you an entirely different experience with variations among them.
Many have been happy with Horizon at Nanny Cay - especially if they wanted a more personal feel to the customer service. I've never used them but have passed thru & observed them in action more than a few times. Certainly a different feel than the very large Big 3 complex.
A 3 week charter (what time of year?) gives you some bargaining leverage to make a deal - the company does not have to turn the boat 2 times during your time. Enjoy
P.S. Might wanna consider
Unused Owner-Time For Sale?