Actually, the setting of both movies are quite similar... but if you'd ask me to choose, I'd prefer 28weeks later cos the story is quite complete. I somewhat felt that 'I am legend' has many unanswered open ends. For eg,
i. Will Smith has been broadcasting over the radio for over 2yrs, if there is really a colony of survivors, then they would have answered to him and get him from the pier which he has been waiting every afternoon.
ii. Sick infected zombies? I hardly think so... cos the zombies are well organised. There seems to be 1 leading the rest. And they have the brain and can think for themselves. They know how to set traps, an eye for an eye, they use dogs against Will Smith just like he send his dog into the zombie hideout earlier. Not to mention that the trap they set is even much superior than Will's trap. Cos they know how to move the mannequin to the open to lure him. Last of all, the zombie have love feeling for each other, else the zombie 'leader' would not be really going for 'revenge' over Will's action of snatching the female zombie away from him.
iii. If the survivors of the infection as claimed by Will Smith that 5.4b against 1.2b of uninfected, sadly to say that the colony of survivors might not be even able to exist. Cos that is really a 5:1 ratio and especially those zombies with such intelligence as above, it is really impossible to stay survive.
Just my 2cents analyse... anyway, both movie are enjoyable.