FastDL or Workshop for server ?

This Depends

If you have like 20 million player models put them in the FastDL and leave everything else out.

But if you don't have 20 million player models then in my opinion in don't really matter.

A problem you might run into is other servers using your workshop collection for there server.

If you have that problem just put the important parts of the collection into the FastDL and leave all the generic addons in the collection.