I pray that it won't be a decision between Scola and Bonner.
I agree that it will take the same type of money, probably Bonner will be a bit cheaper, but as you mentioned, Scola has much more upside.
Scola could be a decent starter at PF one day, something I can't see with Bonner.
if Spurs could sign Scola (for the mentioned 3 million/per year), they should not hesitate to do it.
then the money Bonner would cost, will be better invested in a SF. (if they had not already aquired a SF via trade during this season, what most of us hope).
jury is out on Bonner anyhow, till POs are played. if he doesn't make an impact in the PO, the Spurs won't try to resign him, even if he plays nice this season.
back to Scola: I didn't ever get the argument, that he can't play alongside Tim (because they are both inside/low block players) and so he isn't a good fit. Scola is a very smart player with some range who can very well adjust his game to the other big on the team. right now Splitter is sticking more to the low post in Tau's game and Scola doesn't have any problem with it. in fact he is shooting a higher percentage than in the last two seasons (60% FG).
so I can see him back-up Tim (which would get him let's guess 15 minutes) and play alongside Tim, when Tim moves at C. (likely another 10 MPG)