You can run both, the deciding factor is which weight you run at. Most 30 mm chassis max out at around 390 lbs, after that you would want to use a 32 mm.
You may want to contact Msquared in Utah. They prefer the 32mm and can give you their reasons. For the most part guys run the 30mm though
Like Steen said – It depends on your weight.
I race shifter with 2013 Kosmic Lynx (aka:TonyKart) at 210# with gear and run the 32mm, kart and driver weigh ~420#.
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