Django or Ruby on Rails
I kind of like the Python language
  better and it seems to be more
  full-featured than Ruby for
  statistical, scientific and networking
  (let me know if you think this is
  wrong).

If you think you'll need libraries like NumPy or SciPy, probably best to stick with python/django. I've struggled to find ruby equivalents.

However, the RoR community seems to be
  much bigger

Bigger is of course not going to be better. I've had great experiences with the django community in terms of support, and the pluggable app ecosystem is maturing quite quickly.
I would also add that I find the django documentation to be very easy to follow.
I'm a C#/.NET developer looking to mess around with something completely different - something LAM(*) stackish for building web apps quickly.
I'm thinking either Django or Rails. I kind of like the Python language better and it seems to be more full-featured than Ruby for statistical, scientific and networking (let me know if you think this is wrong). However, the RoR community seems to be much bigger - which might make it a safer bet. Do you think this is important? Anybody have experience with both RoR and Django?

Update: Specifically I'm looking to be able to build a community finance oriented site quickly. There are definitely a lot of Ruby Gems to speed up the process, but I noticed Pinax on the Django side which looks promising. 
I know that Python is already fairly popular for financial/mathematical programming. Anyone else have an opinion?

Update 2: Noticed some comments about ASP.NET MVC. I have in fact done pretty significant work with ASP.NET MVC - a LOB app for Medical Equipment Servicing, and I loved it. It is IMHO a much better and more intuitive way to write web apps compared with ASP.NET web forms. But, I'm really looking for a non-.NET development environment to develop some chops in.