I have stayed at both the Hilton and the Westin. You will be happy to find out that IMHO the Hilton is much nicer. Both are modern could-be-anywhere-in-the-world places.
The Malta Hilton is part of a new complex of shops and residences build around a man made harbor. I was there last year and the residence part was still under construction, but the Hilton was complete. There is a nice large landscaped pool area and the executive lounge is very comfortable with web access and good snacks. The rooms are pretty nice. They are your standard rectangles with balconies or terraces and spacious tiled bathrooms.
As mentioned there are some restaurants and clubs near the Hotel, but the old city is a few miles away. One of the coolest things about Malta are the old busses… you will see what I mean when you get there.