it's just my opinion and doesn't mean anything, but...
a 'flashy' QB who makes plays when plays break down will always get more credit than the QB who is in a system that doesn't often break down. the QB with the operating system doesn't get the credit deserved as he just 'goes to work'... you'll only see his potential or ability when he encounters the rare break down and recovery.
Manziel had a lot more opportunity to show his capability than McCarron, plain and simple, and his ability to scramble while maintaining vision of the field (akin to Baker Mayfield's) is what set him apart... both those QB's had targets that were capable of being covered, and their talent on their feet+vision allowed them to make the best of situations apparently chaotic. Meanwhile, AJ had linemen that protected, backs that blocked or provided the OS dump mechanism, and receivers that ran tight and precise routes- making it only seem that he was 'serviceable, nothing great'... Auburn's Newton had offense sets to highlight his capabilities so he shined, almost in a Randell Cuttingham fashion of play (covered deep? gonna run / spied? gonna pass)...
if the 'quiet' QB's weren't good enough it would have been obvious, is my contention, as they wouldn't have been able to execute the plays asked of them and those plays wouldn't have appeared as smoothly executed as they were... it's the 'fading' into the machine that makes TEAMS truly great, in my humble opinion, NOT the presence of a handful of playmakers that operate almost detached from the remainder of team.
in short, i wouldn't take AJ without the rest of the team, and I wouldn't want Johnny Football on a team built like AJ had.