For a studded tire that is available in both 29" and 27.5" which will perform better in snow/ice all else being equal? I"m looking to start winter riding. My soon-to-be bike, a 2018 Devinci Troy Carbon 27 GX 12S, is able to take both 29" and 27.5" tires.

I prefer 27.5" tires, but would I have more control and stability using 29" studded tires for winter/ice riding? (Assuming tires are the same basic model and width).

I realize that 27.5" studded tires are rare. I can only find 2, by Suomi and Schwalbe.

If 29" tires make for better winter tires, why?
Wheel size probably doesn't matter that much, the one that would probably perform better is the wider one with more aggressive tread or the one with studs.  

However, once you get to a certain width (I'd say above 3.0) then tires can start to more easily wash out sideways on wet snow, which I get on my fat bike regularly, but not so much on my 29er or 27.5+ bike.

You're right though, about having more options for 29er tires with studs.