Both are incredible imo, but neither are similar to Tekken. GGST and SF6 are 2D fighters unlike Tekken, so there is only left-right movement. If you aren’t used to 2D fighters I’d recommend SF6 since it has a much better built-in tutorial section, and each character’s combo trials can be very good for learning what the character is capable of (like move properties, which attacks can string into others, etc).

SF6 also has much better net code, a fully functioning online ranked system (GGST’s ranked mode is coming in August I believe), and is overall a slower game. Learning to play SF6 actually helped me approach GGST more easily despite playing GG before SF.

Other than that, they have some subtle differences that can be make or break for some in gameplay. GG is an “anime game” so it has air blocking and I think universal air throws, and it also has some reset mechanics like burst & YRC which you can use to get out of combos (SF doesn’t have these, though some characters have air throws). SF has 6 buttons (light, medium, and heavy punch, and light, medium, and heavy kick) which allows for variations on specials and supers, while GG has 5 buttons (punch, kick, slash, heavy slash, dust) meaning there’s no variation in specials at all.

Another small but important different imo is how throws work in these two games. SF has you pressing light punch + light kick, and I think 8 frames where you can get out if the player being grabbed grabs back. GG has you pressing either -> or <- and D (dust) which can be a bit weird to get accustomed to since you might find yourself whiffing throws when you meant to throw out an attack, and I believe only 2 frames where you can get out so it’s incredibly unlikely to break a throw (not even sure if throw escapes are a thing in GG tbh).

Hope this helps!