Would rather sell Jones than Smalling. The latter has had probably his worst season at the club, but Jones' overall reliability is minimal.
Both have shown to have the ability in glimpses. Smalling, because of his better injury record, has shown it over longer periods. If you delete this season from memory, then Smalling is a much better player than Jones, but if you include this season, it probably drags Smalling's overall rating down, and probably pushes Jones' up, as he did very well next to Rojo for a period of time.
He's have a few mares this season, but I do think Smalling is still a very good pure defender, it's just a shame his ball-playing is so erratic. He occasionally looks capable of good passes and good runs into midfield with the ball. Other times, he looks like a completely different player, tripping over his own feet and ballooning the ball out of the ground when under no pressure.
I do think Smalling will have a decent PL career if we let him go. I still think there is something about him worth keeping, even if it is not first choice. He has definitely had significantly more good games in a United shirt than bad IMO. Just not this season.