I would discard Chamberlain from 'best 15' consideration. He has had a couple of good games and a few shockers. I do not think he can realistically compete with either Jones or Saffy.
Neville has admittedly had only a couple of good games but significantly that amounts for every game he has played. If he continues with that form, dislodging him from the team is unlikely at best. I acknowledge that potentially, Hill may revert to a Jones/Pyle locking combination, at which point the best 15 becomes easier to ascertain - Jones/Pyle/Higgers/Saffy/Delve. The more interesting and discussion-worthy case is where Neville retains his position in the team.
Assuming Delve and Higgers are automatic selections for the backrow, we need to look at what else we have to offer. As you have noted, Saffy is not an out and out fetcher. As such, we need to look at what else he brings to the team. I would contend that in his case, this is largely measured by his tackle count. However, it is worthwhile noting that Jones is not an underperformer in this regard, with the SMH stats placing him at equal 12th in the league, despite his time on the sidelines. In addition, I would suggest he brings a lot more to the team, including (but not limited to) his ability to ball carry - Saffy continues to run far too upright and doesn't make yards - and moving players at the rucks. This is particularly relevant if we're going to forgo a fetcher, which is ultimately what playing Saffy at openside means, having a better counter-rucking setup is arguably preferable. Also, whilst hard to measure, I've noted a number of occasions where Jones would make the extra effort on restarts to get to the drop of the ball and contest. It is because of all of these things that I would say that Jones' performances this year place him ahead of Saffy's performances (largely) last year.
I've noted previously that this is something of a mental exercise, as it requires a few assumptions. However, if we're forgoing the prevalent Australian balance of 1 fetcher, 1 boffer, 1 skillful backrow then I would contend that you want to have your three best players on the park at the same time, not restrict ourselves to the idea of playing someone who we have mentally earmarked as a "number 7" and not playing someone else because we have mentally earmarked him as a "number 6".
Now, this all may be idle conjecture as Topo has pointed out. Or, alternatively Saffy could develop his fetcher skills at which point he becomes the out and out best option for 7 that we have. But in the situation where neither of those is the case, we lose little in tackling we gain a lot in other areas of the game by playing two large flankers in Higgers and Jones. Alternatively, we know that Higgers has speed, if he can be tasked with applying that in getting to every breakdown, I do not see why he could not assume a (somewhat unconventional admittedly) openside role.