My understanding is they are NOT that close in ability. While Mack is a solid QB, as we witnessed in some spot play last year, Russell is, IIRC, our highest 5 star recruit in history.
The only thing keeping it close is Mack's familiarity with the offensive system he's been studying since his 1st year with CKD at Washington.
Concerning #1, it's about the TD/INT ratio. Every great QB will turn it over, but how many TDs they give you is the bigger stat. I get the sense this is who Russell is. Because of his inexperience compared to Mack, he'll probably turn it over occasionally, but his potential for big plays is also off the charts!
BTW, I don't mean to sound as if Mack can't be a solid QB. I think he can. I'm just so much in awe of Russell's star potential to be the next great Alabama QB!!!