In my experience they feel very similar under the hand, with the Perpetual Starks a bit friendlier to bow than the Spirocore Weichs. I was lucky to get to try the Stark G & D when they were still in prototype and they were described to me by a Pirastro rep off the record as an attempt to make a Spirocore that didn’t need such a long break-in period, and, yeah, I think that’s a pretty decent description.
But, why not both? Perpetual Stark G & D with a Spiro Weich A & E is a great combo—it’s the set I use on my little bass when it’s not wearing guts. Distinctly modern jazz sounding (to my ears) but plenty capable of hanging in an orchestra section in a pinch.