I have the experience of staying at both Regal Airport (2 times) and Marriott Skycity (4-5 times) hotels before. Both hotels are well-maintained, with spacious rooms (by HK standard) and a fair choice of restaurants. Noise was never a problem to me during my stay.
I like the gym at Marriott in particular. The club lounge at Marriott is a bit small though (crowded during evening hours as someone already pinpointed). Never been to the club lounge at Regal Airport hotel so cannot compare.
Remember that once I had a very late checkout at Marriott at about 11pm (flight departing at 1 am) and the lady on the Front Desk didn't charge me that night

So one bonus point.