I’ve been using a Viscacha first a couple of years now. I really like it. Packs well, stable, robust.
I recently bought a 2nd hand Revelate Terrapin (the older style) and used it for the first time last weekend.
Pros: packing off the bike and then loading it up is definitely the better way. It means you can pack under cover when it’s raining too. Fully waterproof. Can be easily removed if parking the bike somewhere.
Cons: not as stable as the Viscacha (though the new version has some changes); deeper when fitted so may hit rear tyre if little clearance; overall slightly heavier; no facility for attaching other gear/Spocket (again, resolved with the new version.
Overall, I think I’d go for the Terrapin now if I was buying new.