I don't think it's fair to compare jspm and Yarn. jspm is an interesting project, but is tightly coupled with SystemJS. Telling everyone to rewrite their apps to use SystemJS and jspm is a huge lift.

Yarn, on the other hand, has focused on npm compatibility from the get-go. The entire point is to be a drop in replacement, with no changes (or maybe very very minimal changes) required to gain a lot of benefit.

In this slice of the industry where everyone is told they have to rewrite to the latest thing every year or two, I find it very refreshing to see Yarn's focus on backwards compatibility, improving existing infrastructure, and having a clear, painless adoption path.