I heard a lot of people saying that they are very similar
Woah now.
SBF is a really subtle, SOL story about a lot of stuff. It has a lot of drama, but it's not angsty or dramatic in its presentation. It's mostly about the relationship between the two MCs, but there are tons of side characters and branching side plots. Quite a few of these side plots have very little to do with yuri or the MCs, and might irritate you if you're looking for a more focused story. I liked it, but it felt a bit typical/rushed near the end.
Sasameki Koto is a sometimes hilarious, sometimes very angsty SOL story also about a lot of stuff. It also focuses mostly on its two MCs, but has quite a cast of characters by the end. The series suffers from a bit of an identity crisis (the first half is mostly comedy, the second half is rather serious drama), but I think it changes tone pretty effectively. Less sideplot madness than SBF (still a decent amount though), far more sugary, and probably a hair behind in terms of storytelling.
They're not really the same type of series at all. I'd say they're about as similar as Hanjuku Joshi and Girl Friends (See: not similar). If you go with SK, I do quite love the April Fool's chapter Dinasty did way back. Either way, you're going to be reading something long and unbalanced, but rather enjoyable, flaws considered.