Personally, I prefer those related sort of command share the same key, but with different modifiers. So, I'd suggest something like:
Tabs to Spaces: Command+Option+Shift+S
Spaces to Tabs: Command+Option+Control+Shift+S
Unweildy, sure, but that's just an example. And just my two cents.
Tabs to Spaces and Spaces to Tabs are without keyboard shortcuts.
Allan Odgaard said he would consider giving them shortcuts if he knew
what shortcuts to use. 
I'm thinking Command+Option+Shift+S for Tabs to Spaces and Command
+Option+Shift+T for Spaces to Tabs. These seem to be available (but I
have no idea how to be sure).
Does anyone have a better suggestion?
For new threads USE THIS: firstname.lastname@example.org
(threading gets destroyed and the universe will collapse if you don't)