Spaces vs. Tabs are generally annoying when editing files, but start programing in Python and this turns into a complete disaster. Simple solution, always insert Spaces instead of a Tab and you are good to go (my personal preference):
set tabstop=4 set shiftwidth=4 set expandtab
If you want to be really fancy:
Will convert all of the existing Tabs into Spaces.
Check out Converting tabs to spaces on the VIM Wiki.