Absolution has a generic mash up up stealth elements while the HD trilogy has its own formula and identity, so to speak. I'm not bashing absolution it's a decent game, but it isn't anything special. Plus I highly recommend waiting until the price is lowered as well if you really want to play it.
The HD trilogy is longer and cheaper too, the better deal IMO.
Hitman: Absolution is a good game and it's my second favourite in the series behind Blood Money, but I think you would be better off with trilogy, unless you're a graphics whore, then you won't enjoy it.