New to MIDI...

Would like to hook Casio keyboard (MIDI in and out) to Aardvark Q10 (MIDI in and out)

Do I buy 3 pin, 5 pin or 7 pin cables?

They all seem to look the same, but sounds like some only have some of the pins wired? Is there one choice that will ALWAYS work? (Like CAT5 RJ45 computer cables... only 4 lines are needed, but if all 8 are wired, it will still work fine?)

Thanks in advance,

Jim