Both are on the water - the Maui water is slightly closer
Both have great opportunities to put your face in the water - Maui slightly better
Both have been very nicely redone
Kehi (just up the road from Wailea) has good and affordable eats (so does Kona, but it's 25 minutes south)
Both are kid friendly (Maui more than Waikaloa)

Big island has an errupting volcano
Maui has snorkle trips to Molikini

Either way, you can't go wrong, dude. I've staued in both twice in the last year.

Only beef - paid to downgrade recently in Wailai (gained a non-existant water view in return for $75 and positioning over the loading dock). Had lost my gold status at the time, nothing like that ever happened prior.