Slightly off topic - how would you compare the 981 to the 987? Are they both manual trans? The reason I ask is that I've been contemplating tracking down a clean, Lapis Blue on tan 981 6M Cayman once used car prices settle a bit. However, I love how the 986 feels and I worry the 981 will be a bit to clinical in comparison.
For me to make a direct comparison is tough. I own both. But it should be understood that the 981 Cayman S is stock (aside from the the Porsche non-adjustable Sport suspension (not aftermarket but added in after purchase), some slightly more track friendly suspension/alignment settings, ECU tune and decent brake fluid.
The 987 Boxster S by comparison, has little left to resemble what it was like stock. Pretty much everything aside from the engine internals have been changed or swapped. It was built to run in Porsche PCA GT1A class. It's pretty much a full on race car, stripped out, caged, JRZ 3 way adjustable suspension, downforce, etc. It's not your "hair-dresser's Boxster".
In very general terms I would say:
Steering (+) 987. It's still a hydraulic system which provides plenty of road feedback. But on track, running under-pressured R-compound tries on an out-lap, it can feel like it's trying to murder you. The steering can shake in your hands like a paint mixer until the tires are up to pressure. That said, the 981 electric steering is wonderful--one of the best systems from that generation. So it's not such a straight forward answer.
Engine (+981). The DI 3.4 liter in Cayman S can be tuned (or un-de-tuned) to increase it's output close to the 350-hp of the 991.1 Carerra. [Edit: Porsche de-tuned it to 325-hp in the 981Cayman S or 315-hp in the 981 Boxster S vs the 991.1 Carrera]
It's smooth, silky, linear, powerful and it sounds about as good as a Porsche Flat 6 can outside of a high revving GT engine. That said, the 3.2 or 3.4 in the 987 is pretty close--it's a GREAT engine, flat-6 magic. But the 981 even stock will definitely show it's legs to the 987--it revs higher (7600 rpm vs 7200 rpm), it's about 50-hp more and IMO, it sounds better.
Interior (+981), the 981 is going to be better. Still no apple car play or android auto outside of aftermarket. But at least it feels like a modern car, though slightly dated. The 987 is starting to feel like it was built 3 generations ago--and it was.
Tough for me to compare the suspension as it's been a long time since I drove a stock 987. And I bought mine fully built. But in stock form, I think the 981 just feels more capable, smoother with higher limits and does it with slightly less drama. I wouldn't classify that as boring. Come off the throttle mid turn and it will still remind you it's a mid-engined car. But the fact that the chassis is stiffer but somehow also slightly lighter than the 987 is a bit telling. The 987 is definitely going to feel more raw. And maybe even more engaging. But ultimately, less capable.
You have any specific questions beyond this, just feel free to PM me.