Django or Ruby on Rails
Like others suggest, trying both is one good way, but really, to me, I would base it more on which language you prefer to use.  Sure, if it's a web app, you'll do a lot with the framework, but "every" line of code you write will be in that language.  Since both frameworks are strong, it's really more a matter of what language you'll be happiest writing code in, day in and day out.  
I personally use Rails and love it, but have many friends using Django.  When we discuss things, time and again, it boils down more to language than framework.  The languages are fairly different, and you'll typically find one you prefer a lot more than the other, at least in my opinion.  I had originally thought Python sounded great, learned it, wrote a few things with it, etc., but it just never "clicked" for me.  When I first saw Ruby many years ago it seemed a bit odd, but when I revisited it via Rails, it just felt so natural.  When I write code in Ruby I feel like I'm pretty much writing English - if I can think of how I think the code should be written, I can pretty much type that and it works.  It just fits my brain better.
Community wise, you'll get a lot of different opinions.  I don't think being "too big" means it's bad, nor do I think the Ruby community is by any means "too big", especially compared to say the Java community.  Ruby and Rails have certainly gone mainstream to some degree, and for those of us who've worked with it for several years, it seems mainstream, but in the grand scheme of things it's actually still fairly small.
Anyway, good luck, and enjoy looking into them and finding what will delight you to work with.
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.