Thanks @irrlichtproject, I'll order the second one then (is it the v3?). It seems neither block the port (neither user, cassette or joystick port for the first one), because there is a kind of extender. But having a more recent version is better anyway. And I need an external because I have two C64 with different sound chips.
Any reason to get the v4 instead? They are 50% more expensive (but with a nice case), I'd prefer to spare money to order a gotek for my Atari ST.