Hi No-regrets,
No experience at all with Spiral Groove.
I replaced the IW with a Brinkmann Oasis, which I really like. Lower noise floor, better articulation of instruments in space, very neutral and natural in presentation with maybe slightly better speed stability, though the IW had no notable problems in this regard. Its ergonomics are simple and elegant, and the armboards are brilliantly engineered with extremely high tolerances. The IW maybe dug a little deeper in the bass.
I made the move mostly because I wanted to try a next-gen direct drive, and I'm glad I did. I wish the Oasis took two arms, though. Its integral screw-on clamp works very well but requires some getting used to: slight differences in screwing tension have sonic consequences, so you have to come up with a repeatable method. And the Oasis needs the Minus-K even more than the IW did. Right on the shelf it sounds relatively hard and thin.
In the same system , I also have a fully-updated Nantais Reference Lenco with two arms, and it might be my favorite (at least sometimes). Great rhythmic grip and drive with plenty of detail retrieval and coherence, even if it has a slightly higher noise floor, is a bit less precise in articulation, and is marginally less neutral than the Oasis with a warmer presentation. Excellent speed stability, too. And perhaps because of its 100lb+ plinth, the Lenco doesn't require a Minus-K nearly as much as the other two decks.
So I'm belt-free for the first time since I cut my teeth on a Dual idler in the '70s. I might return to a belt-drive in the future--plenty of good ones out there--but I'm happy without for now.
Hope this helps.
Best,
Bill