What bass do you currently use for slap/pop? If you use a 4-string bass with a wider neck at the nut (such as a Fender P-bass) then you'll probably want the Wide Gecko 5.
True, the spacing at the bridge is the same for the Medium and the Wide necks; but the wider nut on the Wide neck means that the string spacing at the slap area (i.e. just north of the "neck" pickup) is going to be slightly wider. The neck heel is also a bit wider here too. If you're used to wide spacing at the nut, this will make a difference.
However, if you're currently using a narrow-necked 4-string (such as a Fender Jazz) then you should be okay with either Medium or Wide.
