I have the W5 Pro. There's not much difference between the W5 and W6, definitely not enough to make me want to upgrade. I'm pretty sure my W5 Pro updated to the updated software offered with the W6 - cycling added and upgrades re sleep and activity functions - and the updates work fine on the W5 Pro.
The only difference worthy of note is that the W6 has a rotating external dial and the W5 doesn't, although the W5 has a rotatable dial on the screen so really it's simply down to personal dial preference.
My advice is get the Watch 5 Pro. It's very similar to the W6 and costs less.