What are the major differences between the two as far as compensation, benefits, retirements, QOL etc., over the course of a long career at either? While the legacies (also FDX & UPS) appear to be similar, overall, it does seem that there's a bit of a first and second tier amongst the national/lccs; I could be wrong.

https://issuu.com/aerocrewnews/docs/12-acn-dec-2019

pages 40-45

Legacy is far better over almost any length of career, particularly pay.
Also typically more options as far as equipment and type of flying.

Yup, the others can still be a very fine career (better than most white-collar jobs), but especially if you're young legacies should be the goal. Unless possibly you have family circumstances and absolutely need a certain domicile.

Does that mean there's a certain age or age range at which Nationals/LCCs may make more sense?

Somewhere around 50-55 you won't gain much from all the long-term opportunities (retirements, fleet variety) of a legacy. At that point I'd still do a legacy if I got called, but there is a point where I'd just take the first job you can get and call it good. LCC rarely makes MORE sense than a legacy (if you have or expect offers at both), unless you have very specific geographic requirements.

It's just that there comes a time when holding out for a legacy has diminishing returns.

If you're 30 and take an LCC job, you'll want to keep your legacy apps updated. Unless you have too many DUIs.

Based on that observation, I guess i have too many DUI's then. I took an LCC job at 32, and never had an app into the legacies. No DUI, or anything on my record, 4 year degree. Now i'm sitting 40% in the company. Just because someone is in their 30's and never applied to the legacies doesn't mean they don't have the qualifications. Maybe they are just happy where they are and the legacies is not for them.

Presumably you're a CA. That's a whole different calculus, perfectly logical to keep your seniority since you're already making several hundred grand. I have several buddies in that boat, they occasionally grumble about applying to DL, but they don't actually do it.

I was talking about a (young) new-hire today... HE will not move up 60% in a few short years. Probably leaving a lot on the table if he stays at an LCC if he doesn't have to.