On the resonated Millteks, they have an additional centre silencer (of the resonating function), which is in a similar location to the OEM centre silencer. On the non-res versions, this centre silencer is deleted.
Both Milltek versions use the same rear silencer, and all Milltek silencers (and associated pipework) are a little less restrictive than the OEM components.
But sound-wise - well that is very much a personal preference. And it also seems to be more variable with different engines. The VR6 engine does seem to be a little 'boomy' at certain revs (irrespective of which type of exhaust is used), whereas the GTIs 4pot lump doesn't have those issues.
HTH - but you really need to listen to another R32 with a res version.