Don't know anything about Selwyn really. I'm going to Fitzwilliam (pooled from Emmanuel) so I'll be able to tell you more about it next week. From my experience at interview, and contact with them since, including my college parents, everyone there seems really nice. The tutors who interviewed me were great, and the secretaries and stuff have been too. The place has a nice atmosphere.
It does not, however, look like your typical Cambridge college. The majority of buildings are modern and it is slightly further from the centre than most of the older colleges (this is all relative, however).
My main reasons for applying to Emmanuel were aesthetic, due to a lack of anything better to make comparisons with really. The fact that Fitzwilliam is more modern, some would say uglier, doesn't bother me in the slightest but it might be important to you for your application.
Also, because I'm doing Natural Sciences, which is a really big subject, and in which a lot of the teaching is done in lectures, I think my choice of college was less important than for some subjects. In subjects which have a smaller intake, or where you're more exclusively dependent on a supervisor, it might make sense to look closely at who would be teaching you in the different colleges (a current student would know a lot more about this).
Well I seem to have talked a lot about nothing. Further impression of Fitzwilliam next week, if you like.