I think taking the LW and being the only 3 division champ is the biggest thing he can do for his legacy.
Years from now when the MMA fans have never seen Fedor, GSp, Anderson, Jones, fight and they are just reading stats that would stand out far more than becoming the 4 time WW Champ in a division he is already GOAT in.
But I would say this. If GSP was able to win the LW belt and he still felt he had some fight in him then he should announce as his last fight, Woodley and the WW belt. To show he was able to move across three division and win belts and to derail Woodley's chance of chasing him down as WW GOAT would be worth it.
I honestly think that type of legacy 3 Division Champ (and P4P GOAT by virtue of being able to fight up and down) would cement him as GOAT of all of MMA likely forever. But Woodley would be a dangerous fight. Another Hendricks for GSP and we know how lucky he was to get away with that one.