You are moving from DLP to LCD, that is really the shift you should be concerned with. The better projector is the UltraBlack model, I wouldn't go for anything less. However, if you are going to get an UltraBlack you should consider the low-end JVC that is now out and blowing everyone away, of course your'e above $2 grand now....is it worth it to you is the question.
Right now, no "budget" 1080p LCD can produce a better picture than this Marantz....at least nothing I'd be willing to spend money on and then go "OMG WHAT A BETTER PICTURE!", it might be "yeah thats a little better", but you're only going to get $500 or so for a used 720p DLP at this point (regardless of quality) so you figure if the "upgrade" is tangible enough for you to spend the bucks or not.
I have a Marantz as well....I think I might have 300 or 400 hours on it, like you I'd use it for movies....the only problem I have with it, which is the same for ALL digital projectors, is that dim scenes always look muddy..and I mean dim "lit by a flickering torch" scenes, not like...Escape From New York or what have you...and there is no way around that, all digital projectors tend to suffer this same fate. So why upgrade?
1080p is nice but not so nice that its going to dramatically alter your viewing experience unless you are doing 1080p gaming from a PC, lets say....
IMHO of course