It's...a prequel.
I agree that it wouldn't hurt to do Breaking Bad first. Otherwise you might not get things like Krazy-8 or even the "working in a Cinnabon in Oklahoma" reference that kicks it all off.
It IS a prequel, yes. But it has so many callbacks --- call forwards? --- to Breaking Bad that it would be a shame to miss all of that context.
It also begins before Breaking Bad, but allegedly ends in a world after Bad was over. So in that way, it's almost as much a bookend as a prequel or sequel. If it does, that is.
One thought.... if Breaking Bad hadn't existed or if you watched Saul first and then decided to follow with Breaking Bad would it have the same impact? If Bad hadn't existed, I don't know that Saul gets six seasons. I kind of think the audience for it might have been bigger had it been first, though. It's a really well-written show, solid acting, solid story. The way it's shot is really fantastic. The camera angles, the visuals. Even simple, throwaway shots like looking up at a pissed off Hector from beneath the pool water. It carries that same sense of framing and style from Breaking Bad that was so fantastic but it's (in my opinion) even better.
Another thought. It's 'Better Call Saul' but it's really much more than that.
It's Gus Fring's story.
It's Mike's story.
It's Krazy-8's story.
It's Hector Salamanca's story.
It's Tuco's story.
It's Saul's story.
All of them were contributors to Breaking Bad
But it's also
Chuck's story.
Kim's story.
Howard's story.
Lalo's story.
Nacho's story.
In the time it has, in each episode, it weaves components of all those vastly different but intertwined stories into a single cohesive narrative.
I don't mind admitting when I was wrong and I was. It's a fucking great show.