We've flown Allegiant several times. Very reliable and affordable. What you need to know:
-Boarding early is extra
-Checked and carry ons are extra. Prepay at time of booking.
-Beverage and snacks are extra. Buy a drink before, and carry on snacks.
-DO NOT use a debit card to pay, even though they offer a $4 per segment discount. You lose all CC protection.
-Very reliable and on time when we've flown, and we have always had friendly staff.
-Schedule is very limited. If it fits your schedule, then go for it!
-If you book your rental car with them as part of a package, there are usually significant discounts. Con - you pay for the car rental in full up front, and then the additional taxes when you pick it up from the rental.
-Sanford airport is much easier to get around and through than MCO, especially if with younger kids, but is a longer drive to get to Disney and other areas.
Ultimately it's up to you, that's just what we've experienced before.