Sell Canon 17-40 for Tamron 24-70 f2.8 VC
Any lens can sit in your bag for the chance you might use it someday. Don't become a gear hoarder, get what you suits your current needs the best. If the 17-40 is collecting dust and there is another lens you will get more enjoyment from, then sell it. The 17-40 is a popular lens, you will have no trouble finding another one down the road should you need it again. By that time there may be something better and cheaper.
The Tammy 24-70 is a fine lens that sounds better suited to what you need today.