I mean if you already have photoshop you may as well keep learning it. It is the industry standard for a reason (partly because of corporate greed but that's another discussion). The biggest negative is that it's a subscription model, but so is Clip Studio on a Ipad. Everything is going to go subscription eventually, which is a shame, but that's the direction programs are going. (and if you are a student you get a very big discount so look into that)
And the reason I like it is because it's just a lot more powerful and I make some very large and complicated drawings sometimes. Clip Studio crashes a LOT and has quite a bit of slowdown. There are things that Photoshop does that really helps my drawings that Clip doesn't have yet (maybe it will one day) and there's things clip has that Photoshop does not have yet (but again, maybe will eventually). And like Clip is meant more for one style of comic while Photoshop leans more towards photo manipulation and painting. So it also depends on what you want your end product to be.
I haven't tried SAI but I do have plans to get it because it can do some really beautiful work. Especially in Asia people are just killing it with SAI. But, from what I've heard, it isn't as intuitive as Photoshop. It's actually it's own level of difficulty. But, I've also heard it has a very unique feel and does linework and painting in a cool, unique way that really clicks with certain people.
Thing is, the Photoshop setup looks a lot like the Clip Studio set-up. So if you're hoping to find a simpler program...they are mostly all trying to copy off of Adobe since that's the industry standard right now. So, if you learn Photoshop, and you will also learn how to use most other painting programs.