I’ve never used either of those models, but the FS76 looks to be from the same “era lineup” as the FS72 trimmer we bought new in 1992. The FS72 has been running the same fuel lines and carb diaphragms that whole time, with about a zillion hours on it. Only this year did it show signs of needing a carb kit.
We did munch one lower gearbox (found a used take-off replacement) and I had the muffler off once—once!—for
cleaning and it wasn’t bad.
It has run a variety of two-stroke oils mixed at 32:1 throughout its life, and been used pretty hard. It has been a very reliable tool. I expect it will continue to be with a carb kit and new fuel lines this year.
I would talk with a Stihl dealer about parts availability before you make a decision . Also, why not keep both? You might need a backup if one is down for repairs. Or you might have a buddy help out sometime and a second machine could be useful.