I moderate a top 100 sub and if the built-in stats are to be believed, the recent breakdown is something like:
= Pageviews =
40% apps
28% old reddit
17% mobile reddit
15% new reddit
= Unique visits =
41% mobile reddit
27% new reddit
24% apps
8% old reddit
This doesn't really capture the ratio of contributors who use each (or perhaps, more interestingly, number of contributions per platform), but it's hard for me to guess that accurately since new-reddit detractors are a very vocal group.