I'm been on Fet since it's first year as NookieNotes.
It's intentionally not set up to be a dating site, so finding people to connect with is much more difficult than here. It's not gonna show you who viewed you, and most of the profiles have no stats.
The messaging system is better. It's easier to share content on a feed, photos, or writing. It's easier to learn there, because writings are more discoverable. Because it's really a social site, like a kinky FB.
There are MANY more local events to get out and meet people, at least in my area (we have a super active kink scene). Munches (gather for food), sloshes (gather for drinks), mixers, educationals, parties, etc.
Fet is mostly free, and totally free to message, so you know the people there have a lower bar for entry.
SLS is, like most swinger sites, mostly heternormative, mostly cis, and very couple-centric. FetLife is a variety of everything: ages, races, sexual orientations and genders.
And, of course, Fet is not all about sex (although there is plenty of that in SO MANY variations), but about kink, which could be getting cake smooshed into your chest with pretty bare feet, spanking, power exchange, even suspension by hooks.
Not for the faint of heart. But definitely for the adventurous.