Filament sizes

Well, another friend is building a printer, I'm providing parts and he wants to use a commercial extruder and use 1.75mm filament. I'm going to have to do a custom design to mount the extruder to the i3 so he's shipping me the extruder. I don't have any 1.75mm filament, but I suppose that it's inevitable that I switch, the whole world seems to be going that way, so I ordered some up.

If I like the extruder, I will get one for myself. I'll still have to have a 3mm extruder for months if not years to use up all my old filament, but switching is probably not a bad idea.