I've barely touched my 50mm 1.8 since getting my Tamron 17-50.. and yours is the better one with the vibration compensation. That's a nice piece of glass you have there... the 70-200L as well.
If you're going to go with a 50mm prime may I suggest the Sigma 50mm 1.4? It's $500 but I've read countless statements proclaiming it's super sharp even at f/1.4 (something the Canon 50mm 1.4 is not).
Unless you want a cheap piece of glass you can break and not care about, then get the nifty Canon fifty. Beware though, it sucks at auto focusing (at least mine does).