I honestly don't like either right now. Tekken looks boring and more of the same crap we've had for several generations now. Graphics aren't carrying it like PS1/PS2 days. SF6 has one of the worst art styles I've ever seen. The graphics are clean and animations are good, but the characters look like they're created in a porn VN engine and then somebody comes along and says "now fuck my shit up" to the faces. The game modes and gameplay look pretty good though, so if I had to pick one, I'd pick SF6.
My real answer is GBVS Rising. It's got a ton of tools to level the playing field between vets and new players, the character designs are great, the gameplay systems are interesting, it's got a great system for training new players while giving them more or less a level playing field from the start, tons of single player content, and now Rollback netcode.