I think the two main differences are that Cashu is not a federated system and it has no on-chain support (yet?). To be clear, I freaking love Fedimint!

I haven't spent too much time thinking about tradeoffs but there is clearly an improved trust relationship with the mint if they are federated. On the other hand, a non-federated approach is a lot simpler to implement, which means that it should be a lot easier to add to existing Bitcoin applications.

The simplicity of Cashu (and the standardized client protocol we're currently working on) should enable other devs to build their own clients and thus make it actually useful.