Thanks for your answer!

I decided to go with the non-MIPS Giro Foray in the end. Fluo Yellow color for added safety!

I did some research online, and this article by this group that does a lot of research on helmets sums it up pretty well for me:
MIPS and Sliding Resistance of Bicycle Helmets

Basically they say MIPS has not really been proven to work, it might work. It might not.
The tests MIPS do involves dummy rubber heads that fit very tightly around the helmet, so the extra slip plane really makes a difference there. But a real head already has some 'slippage' between the head and the helmet (hair, scalp, air space,...), so the added benefit of the MIPS slip plane is not certain. This makes sense to me, as pushing any helmet on your head will allow it to shift a couple of mm.

Anyway, not any of the helmets I liked or could afford had MIPS, so I went for the other option. I'm not saying it is not a good technology, but not significant enough for me to alter my choice.

Maybe MIPS will make its way to all helmets in the future, and I might upgrade then.

Hope you enjoy your helmet, and keep safe!