Hi Glenn,
From a diving perspective it would seem an easy choice - the diving around Mactan is okay, certainly not bad but by Philippines standards would only rank as okay. I've still not dived Malapascua (I really need to get that corrected soon!) but reports always make the diving there look exceptional.
The better dives in Mactan tend to be advanced or tech dive sites where any recreational NDLs only give you a few minutes of bottom time. Malapascua seems to offer great diving for macro, wreck and of course the threshers/rays.
There is a 'thresher cleaning station' near Mactan (just off Olongo island - Tongo point or something I think?) but it's more a chance of sightings than the very likely sigtings at Malapascua. I've been to the Mactan 'thresher' site twice in the past few years and can't say I got to see much - certainly no threshers, plus it's at 40m so once again recreational dives there will be very short!
If you're looking for nightlife then Mactan is liley to be the better option, although the Hilton probably isn't the best location? Maribago tends to be a better option for diving if you're planning on diving from Mactan - places like Maribago Bluewater would give you a more relaxing and pampered stay than the Hilton and the price is similar if you book in advance with Agodo.
If night-life and pampering is less important than diving then I'd say spend the whole dive time in Malapascua, and allow a single night or two in Mactan to relax/enjoy on the way back!
Hope that helps,
Karl
