I think Rondo's the better pure player.
But it gets a little more complicated than that when you think about other stuff. I'm not sure either one is good enough to be the A#1 option on a title team (Conley definitely not, and I think Rondo is too deficient in certain areas to be so as well; his shooting and FT shooting problems are real real problems), and I do wonder if Conley's skills (specifically shooting and FTs) mesh more naturally with the other random players you would need to build a title contender considering neither PG is that A#1 guy. In other words, I wonder if Conley meshes better with the A#1 guys you need on a title team.
A few other things too:
-Conley is 2 years younger.
-Conley makes 8.2, 9, 9.6 million then is an FA
-Rondo makes 12, 13, then FA one year sooner
-Rondo's next contract is more likely to be bigger as well. -There are always small question marks post major injury.
So, because Conley is cheaper and not as good an overall player as Rondo, this means 2 potential things:
-When you take just Conley over Rondo, you have about 4 million extra dollars of nba salary to work with. Might make a difference; can be difference between MLE vs MMLE for example.
-If there were to be a trade, it would likely not be "straight up", but would include some extra stuff from Memphis. So the choice may not be Conley vs Rondo but rather Conley + 1st rounder or Conley + Ed Davis for Rondo.
Makes it more tricky.
If I were building a team from scratch and one of these two had to be my first player and I had to win as many games as possible in that first year, I'd probably grab Rondo at his 12 million. But if I had a good team that lacked a PG (say NYK, Memphis, Miami, OKC) I might take Conley so that those teams with other good players who are comfortable with the ball don't get their spacing messed up by adding a ball-dominant non shooter.