but, is it a flat belt or a square belt?
in general, for flat belts, lower thickness can provide better wow and flutter but if it's lower thickness than the original you should consider to get it also a small bit shorter so that, in the end, you get more or less the same tension as a slightly longer but thicker belt.
if it's square and its cross section is too small it might apply too little tension and not making the work properly, even if you choose it shorter.
But, in any cases, if those belts aren't expensive, I would try to get a few different options, try them all and then choose which works best, by also trying to measure wow and flutter and/or checking speed stability throughout the whole side of the cassette.
I know it seems complex... because this is a complex matter, actually.
