Here's the code and results ...
As you can see, the histograms are pretty similar. For this particular Rails project, I was using HAML, so I'm not sure if there'd be any differences if you used erb or not. Also, as I noted, this was a fairly small project.
The code runs against a single file that's passed in on the command line. So run it like this ...
ruby letter_freqency.rb moby10b.txt > moby_results.txt
for example. For a rails project, I cat'd all the files together and then ran the code like this ...
cat `find . -iname \*.rb -or -iname \*.haml` > rails_files.txt
ruby letter_freqency.rb rails_files.txt > rails_results.txt
Let me know if you try this on one of your projects and post the results in the comments.