I'd probably say that I'd find HL2EP3 more fun than L4D2.
However, L4D2 is obviously repeatable without getting boring, and what with the new stuff they've got, it should be some great fun.
I'll enjoy HL2EP3 more, initially, but it would get boring without having something like GMod to play with it afterwards. L4D2 will provide some good fun for longer.