When the 11th round rolls around, I guarantee you won't have a second thought about Murray and the value you got for keeping him. Go ahead and draft Welker in the 3rd if you want, WR is deep enough this year it may happen. If not you will be able to get a WR pretty similar. Zero chance of getting a RB of Murray's talent in 11.
Murray should be by far your first choice. 11th and 12th round is pretty much a wash; when healthy, Murray was a top 2-3 RB, and if he stays healthy he certainly has the upside to be one of the top fantasy performers for years to come. If you can keep the players year after year you have to consider Welker as your 2nd keeper. Sproles in the 12th is crazy value, but as you said Welker is so dependable. Really up to you, but letting Murray go would be a mistake.