Personally I would recommend starting with SP, then move on to co-op. The MP system is hugely flawed unfortunately - specifically the user created map lobby system (to name one such flaw).
The map editor, however, is terrific. It is much better as a SP style map creation tool - which can then be played solo in 'test' mode - great fun since you can place enemy AI, vehicles and wild animals which oddly cannot be placed into a MP map. (that never made any sense to me!!)
(Just my personal opinions)
"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein (1879-1955)