The idea of each player having his own mod is not that good. There will be loads of mod abuse and you will se f**ked up mod parts and will not distinguish a default weapon from a default weapon modded to its last pixel. Imagine how many image files you'll have after a month of playing with hundreds of new players each day, each of them having his own mod(s). So, F11.
Still, I think that the idea of downloading a Mod from the server and use that mod only on that specific server is pretty good. So, F12 -only if you'll have an option for auto mod download. I think it would only add more juice to the fun-jar.