I have held both positons: SRE, and Software engineer. SRE workflow in my experience has been break/fix. Your job as SRE is interrupt-driven. You must put out fires while promising everyone that another fire won’t ignite for similar reasons.

Software engineering is more structured. You estimate an amount of bugs/features you can deliver, and work to make it so. You have some extra padding from the fires, but the fires can still change your priorities from features to bug fixes and better testing.

I prefer Software Eng to SRE because bugfixing/testing over new features doesn’t bother me, as where every new fire as an SRE was painful and sucked the life out of me.

As SRE I always had some software dev work I hoped to do, but never could get into that zone because of the constant fire fighting.