I've stayed at both. The rooms at the Airport-Downtown are much better. And they upgrade reward stays. The water park at the convention center is the only reason to consider staying at the convention center. If you are willing to drive between properties, i'd suggest staying at the downtown HI after playing at the convention center's water park.

The convention center's water park is an extra fee, even if you are a hotel guest. And non-hotel guests can be admitted to the waterpark for the waterpark fee.

That said, the Airport-Downtown HI has a respectable waterpark which is included for hotel guests at no additional cost. Each room is given 5 wrist bands, so we invited some local friends to play at their water park for the day. If your kids are 7 and younger you'll be fine. If you have older kids, they will like the convention centers water park better.

hope this helps!