5 months to go and you haven't decided? I'd say you are past V1 and you're not sure what to do?
I don't know where you are in the "planning" stage hence my ? mark.
I can't dig out the spread sheet that I came up with more than a decade ago but I'd say the 20 vs. airline $ difference is over $1 million but there are MANY variables to consider as you can imagine. Couple of basic things to think about
: Can you guarantee that your last assignment will be a flying assignment approaching 20? How will the airline hiring be then? How about the future economy? Upgrade times 9 years from now (if you care)? These are unknowns that can bite you. Currently, the airline hiring is in full gear; knowns vs. unkowns...you see where I'm going with this?
As for current upgrade timeline, it's anywhere from 8 years to 18 years depending on which legacy/major airline. That's just my guess. There are a ton of "high time" civilian folks out there and military guys are not in that club, though ex-mil types get a special pass with less hours, especially the single seat/fighter types. I'm just sayin'.