Excellent question, mon frier!

So far, Gears 2 is a lot better than the original. It has much more depth in it's story, more ranging landscapes, and overall much better quality to the whole thing. Alos, it's equiped with a set of good achievements, including some like in the first game (kill so many with such and such), but without the limits of being online. By this, I mean ANY game (campaign, online, offline, hoard - sorry, no explanation for hoard right now, but it's fun) all adding up to one achievement. And all the required is scaled down (like from 100 kills to 30, or 10 a certain way). The only exception an achievement called Seriously 2.0, where apposed to killing 1000 enemies, it's 100,000 (meant for online play and campaign plus the to be explained later hoard). 

To Dead Space, I'm not that far in. Chapter 4 of 12 (in Gears I'm in Act 4 of 5, with about 7 chapters per Act)But, I can say it is freaky, and a lot of fun. Kinda like a F.E.A.R. - BioShock - Resident Evil fusion, with a setting and story relative to Alien. Anyway, it's great too.