As above you may wish to visit them to get a feel for the universities. The major factor is probably going to be the prospect of living in London vs not - it's not for everyone. They're both good courses - Imperial perhaps more "prestigious" but I haven't seen any evidence that it's better or worse, other than this fact, than the Southampton course.
The people I know who did aerospace at Soton (weirdly quite a few due to moving into a houseshare of them way back when) all seemed to enjoy the course, and it seemed to have a very good community feel for those on the course. From what I gathered as well as generally getting on very well as a cohort and not being so much just "bunch of people on the same course" the lecturers were described as very congenial and often chatted with students "on the same level" so to speak. However, they were on the course quite a while ago now, and it's possible the environment has changed somewhat. I think the course itself has gotten bigger at least.