For a studded tire that is available in both 29" and 27.5" which will perform better in snow/ice all else being equal?
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.