If you think Ryan finishes 5 pts a game better, that's an 80 point swing.
If 70-80 is average weekly score, that's a pretty significant difference.
I don't think their points will differ by that huge of a margin so I'd go with Luck in the 12th...Based on ADP Luck is a huge bargin while Ryan is being taken 60 overall. You could potentially score him in the 6th and have both QBs again. (I assume it's a 10 teamer like the Ridley post).
If you think they will have that huge a difference though, I could see keeping Ryan and settling for whoever late, QB is deep this year. Seems to me you aren't as likely to see an 80 point swing in points with whichever RB/WR you'd take in the 5th so I'd go with my gut if I were you. Also in a TD heavy keeper format QBs might go earlier than usual, idk, so that's something to take into account too. Would probably help to look back to past drafts and look for trends there.
Also another aspect to think about, if this is like the Ridley league where you keep them 2 rounds higher the next year, Andrew Luck would give you a year in the 12th, 10th, and 8th. That's potentially 3 years of great value, another thing to consider. But idk if this is the same league or if you are in 2 TD heavy leagues. Just trying to cover all bases