Doesn't Antsport mock drafts run a compilation of various draft types? That may be a good place to check into for comparison sake. Even so, Manning's 30, that gives him at least 6 more seasons of performance at this highest level. If you take a RB in his prime or pluck him right before it, he's probably around 24 years old and generally speaking, poop out by 30, which is the same 6 years. Why not take the QB? Especially in that type of scoring system.
I like simple pleasures, like butter in my ass, lollipops in my mouth. That's just me.