I'll let others argue over which is better. As Stu said, the main difference between the two is the metal vs. plastic parts internals.
However, I have a Nilfisk (C110) which is perfectly adequate and has so far lasted about 5 years or so.
In terms of lances, they're all the same internally - the only two things that ever differ from one brand to another is the nozzle adjustment shape and/or colour and the shape of the bottle - so I'd always recommend going for the cheapest. The cheapest I know of id from Direct Hoses, whilst you're there treat yourself to a 15m quick release hose for you new pressure washer from there too. You'll thank me for it.
Also, commit to a pressure washer before ordering the lance and/or hose; Karcher have two fitments depending on which model you go for, Nilfisk just use the one, not so sure about other brands.