FastDL or Workshop for server ?

If you're going to host a server, and you don't have something like a 100Mbps+ synchronous line, then going with the Steam Workshop is a much better idea.
If you have some custom mods and things that can't be added to the Steam Workshop, then you can use both a Steam Workshop and a FartDL server.
It's always better to use Steam because if you package up things like custom textures, models and other things that you have downloaded from sources other than the Steam Workshop, and upload them, people with fast internet connections, often faster than the server can join far quicker than if you have just a FastDL server.
Also -- Depending on the country that someone connects from, it's going to be a lot faster because Steam has multiple places around the world that mirror downloads from the Steam Workshop.
If you're connecting from another country your download and upload speed is not always going to be exactly what you have in the local area or country.