The game is fairly short but the slower tatical aspects make it seem a little longer then GoW. I easily finished both of them in a weekedn.
Also, (like I originally thought your question was in the title) the ending of GoW is much more detailed and satisfying then the ending of RB6 just due to how the very end plays out (IMHO). Although, RB6 does make a little more sense as GoW seems to be missing a cut scene telling us why we need to go where we go and why the item in question is where it is and in Locusts hands.
RB6 is a great game and worth a play through on normal and realistic and then there is the Terrorist Hunt mode that adds some playtime if you want to go through it. As far as I can tell most of the 360 shooters have been about the same length, give or take a couple of hours or so.