my friend had the same prob with these two universities! he chose Hertfordshire has first choice. Personally, cause i go to that uni a lot to hang around with friends i reckon it is brilliant, facility wise. Really good gym, Galleria nearby and Font bar (student union bar) is quite good. There is a cinema nearby as well. You can get the train down to central london in around 30-40 mins. the SU have a lot going on during weekends, and a quite a few socities to join.
Although academically it is not that great, but it is improving a lot with the new campus and buildings which are structurally brilliant! The new library is good with a lot of computers and space to work!
i really dont know that much about westminster though sorry. My cusin went there for law. Apparently it is not that great. The buildings stink (lol) and my firend went their for an open day and found it quite crap. however it is close to baker street, where Maddame Tussuads is and quite a few bars are. IT wont take long to get to Piccadilly cirus, leicester square and covent garden.
Accademically herts is better, according to the times.
for law herts is better then westminster.
and westminster dont to economics according to the times????????
overall (not being biased) but i would choose hertfordshire.