From the perspective of consuming the feed from a custom-made Roku channel, I don't think it matters - and most people prefer JSON to XML these days.
MRSS is a standard though, so if you wanted to be able to plug it into other software you didn't write (or have other people use the feed in their software without having to document your format) then it could be a good choice.
(FWIW I'm parsing hundreds of items from JSON feeds into ContentNodes and plugging them into RowLists with no noticeable performance issues.)
