Most WN wraps have PUL (plastic-type material) on the inside, so the outer is for show rather than functional! Only thing I'd recommend is that you avoid cotton outers if you have a heavy wetter - it can wick moisture from the nappy. I personally love minkee and cuddlesoft outers with embroideries! They also do fleece which is quite good for nights but not good for day - if your LO is sat or laid in the same position for a length of time, wetness will be forced through. I don't use fleece any more.
I recommend wool with night nappies; it allows moisture to evaporate. We now exclusively use wool at nights. WN don't do wool but lots of WAHM's do, Ali (Babyroo on here) has lots of fantastic wool in her shop; Pixieknits.