Gears 1 sold over 5M on a smaller user base than what the sequel will have, but then again there was less competition back then.
MGS3 did 4M on the PS2, and the sequel is on the PS3... Beating Gears 2 isn't completely impossible, but very unlikely.
As for preferences, I'll probably like MGS4 more. Shooters are fun and everything, but I've come to vastly prefer ones with something else than just shooting over the years. Sneaking and shooting wins over just shooting from cover in my books.
Reviews will probably be roughly equal. MGS4 is definitely in 90+ territory, and while we don't know much about Gears 2, it probably won't do substantially worse than the original.