Availability and amenities. It seems the Hilton (at least the one near the convention center) is quite popular -- for my last convention that would have been my first choice, but the convention block was completely sold out and I could only diamond force 1 room (and I needed at least two).

The Hilton is going to look like a Hilton, walk like a Hilton, and quack like a Hilton so you're going to pay for things like quality room service, lounges, concierges, and a premium for premium service, etc. The HGVC has no lounge, lesser dining, I don't recall seeing a concierge. The Hilton is also basically attached to the OCCC, which allows the "location, location, location" premium for convention travelers; the HGVC is either a decent walk (with a free game of frogger on I-Drive) or a shuttle bus ride to the OCCC, making it far less convenient for the same demographic.

To answer your other question... it depends on where in Orlando you want to be. Near the convention center/International Drive, I've had great stays with the Embassy Suites, and good enough stays at the Doubletree "near Sea World" (though, IMO, the tower rooms are much nicer than the outside corridor rooms).

On the HGVC side, I was underwhelmed by HGVC Las Palmeras but would stay again [I would not however, trust the bell staff with anything based on service failures there] and several years ago I was so disappointed and underwhelmed with the service and quality of the room at the HGVC Tuscany Village [my first experience with any HGVC property, so that may have slanted my perceptions] that I went out of my way to avoid all HGVC properties for a few years...I'd hope it's better now, but I'm extremely unlikely to ever return to that specific property.