If it is not a race and there is no steep climbs, sustained power is probably better. The reason people suggests short power to increase sustainability is because you would be able to surge more often without burning out.
A little bit of a mixed question, since focusing on any plan will help increase your FTP. However seems like the question is really “how do i go further, faster without fatiguing”.
Sounds exactly like my previous training. For a century and endurance ride, I’d suggest the sustained power to hold power for extended period of times. Will prepare you very well for that since it’s a stead state effort and heavily focused on muscular endurance - can speak from experience with this one.
The short power plan definitely has its fair share of focus on long intervals (ie baird peak), but bakes in more shorter, 2-3 min 120% FTP intervals. I recently switched over to this to keep up with my buddies in our weekday world championships, which is a punchier/aggressive style ride.
I’d suspect if you haven’t done a century yet or completed a full TR plan focused on that, I’d start with the Sustained power and see where that goes. It’ll certainly boost your FTP as well.