well, lessee…
Felix is more talented. He's also notoriously brittle, lackadaisical and inconsistent. Phil Tanner is somewhat less talented, but dedicated, hard-w*rking and apparently durable. The Cowboys like Tanner. They have to keep insisting over and over that they still like Felix as it's pretty obvious from their actions that they're lying and no longer do. Given a long-term absence of Murray, I think it unlikely Dallas would be satisfied to open next season with Phil Tanner entrenched as their featureback but with their current talent in place I'd give odds on him winding up as lead-dog this year.
In a somewhat similar situation, I give you the Pgh Steelers. The Steelers have been dressing 5 RBs for games: a FB, new-toy scatback Chris Rainey, and Mendenhall & Redman as RB1&2, thus leaving one slot for Jonathan Dwyer or special-teams-captain Barron Batch. Dwyer's been sitting. Last wk, with Mendy and Ike sidelined, Batch was the only back active who could fill an every-down role. Virtually everyone in the FFB arena seems to have concluded from that situation that, should the top two be unable to go, Batch would be next in line. I am amazed that nobody's yet asked Tomlin about it. When they get around to doing so, I'm certain that he'll indicate Dwyer's still next in line for starter carries. There has been absolutely no indication otherwise from anyone in the organization.
The RB injuries are really flying under the radar at this point, but it's entirely possible neither will be ready to go. Should this be confirmed, my money's overwhelmingly on Dwyer over Batch to get the start & the majority of touches. If you wanna speculate, now would be the time to do so n' avoid the inevitable rush…