I use er-32 collet chucks to machine d2.
I have end mill holders but don't use them.
I've started using shrink fit holders and vastly prefer them to either collet chucks or end mill holders.
In theory collet chucks have advantages in finishing and end mill holders in roughing (see cnccookbook.com for a discussion of this and pointers to the reference research).
If I had to pick one or the other it would be collet chucks. None of my end mills have wheldon shanks anymore.