I think the real problem with comparing these two is that "28 Days Later" is an exemplar of its type, but a late-comer to the field, while "Alien" in many ways defined a lot of its own genre. It's like comparing Babe Ruth to, um, a really good modern ballplayer. The modern guy might even have better stats (IIRC, all of Babe Ruth's records have been beaten), but he's a polished latter-day version, not one of the Big Names. No one in later years is going to think of a given zombie flick as being in the style of "28 Days Later," but there's a whole aesthetic to Alien that has spawned innumerable copycats, knockoffs, and a whole little subgenre of scifi/horror.