Trailforks data is mainly fed from Strava, however in some areas the trails are curated by trail pixies, land managers, local clubs, etc. Areas or trails can be blocked out if particularly sensitive (a good thing).
Some places with popular trails, Strava has dozens of segments for the same trail, but Trailforks may be filtered to one or two.
Trailforks has nice ability to report on trail conditions and features or issues. Strava is just about KOMs.
Segment explorer in Strava is garbage. I don’t really know how it filters things but it only shows a fraction of the segments that Strava has.
As for “which Strava”, if you refer to the paid packages… free. Until Strava acknowledge mountain biking as an activity and split segments so roads near an MTB segment aren’t populated with roadies and vice versa.
Unless you don’t care about leaderboards (like me), then it doesn’t really matter so much, except you can still see segments you didn’t ride on your activity due to matching a road or MTB segment. It also makes challenges and club leaderboards pretty irrelevant unless you know everyone else on there did the same type of riding.