304 stainless is pretty corrosion resistant, my concern was if they used 400 series stainless which can rust. I suspect using stainless is more expensive than brass, but that's about all I can think of. I'm not a metallurgist, but both my brothers are - I'll shoot them an email to get an opinion.
Edited to add a quote from my brother the expert.
The only technical point I can think of regards salt. Type 304 SS will pit in a saline environment, so I suppose much depends on the seal or boot configuration to exclude the environment. I also don't think that any 300-series stainless steel would hone very well. There is a galvanic series difference between either brass or stainless and cast iron but that doesn't favor one over the other. Again, it relates to exclusion of salt water.
Based on what he said I think I'd prefer brass. A collector car won't see much salt, but at least where I live there's still going to be some residue left on the streets in the spring. If you live near an ocean it's also a concern. His point about honing the cylinder should be considered too.
Edit X2: Gray iron, 304 stainless, and yellow brass all have different coefficients of expansion.
Gray iron 6.7 micro inches per degree F
304 stainless 9.6 micro inches per degree F
Yellow brass 11.1 micro inches per degree F
(one micro inch = 0.000001 inches)
This means the sleeves will expand more than the brake cylinder body as they heat up, although I don't know if it's enough to make any difference either way - even if the brake temperature gets to 400 degrees, the difference between iron and brass is still only about 0.001".