The season is still relatively young, but KJ has yet to put on the court the kind of talent I'd expect to see from a top 5-7 draft pick. If that is his goal, then I'd suspect he is the most likely of the two to return. And that is mostly based on perception. The perception is that he has yet to fulfill the lofty expectations that top 5 talk would elicit.
Springer, on the other hand, has likely outperformed, to an extent, his expectations, slightly. The questions on him surrounded his outside shot and perhaps his quickness. I'd still guess his best NBA position is likely at PG and that he'd be first-served getting a chance to run some at Tennessee. I think he's a much better fit there than Keon Johnson, but Rick Barnes, to this point, disagrees, so what do I know?
Based on what we have seen in 9 games, my bet is that Springer is the most likely of the two to leave, but a lot can change over the next three months. Both guys are just scratching the surface, and it would seem that both are knocking on the door of bigger roles based on what the coaches are saying publicly.