It's generally considered if you're going for the cheaper range then opt for nilfisk due to the metal internal impeller, whereas cheaper marchers are plastic until they get more expensive.
The nilfisk centinnal or C120 is around £100 but can often be found on sale from Argos/home base as low as £70
As above really, I would avoid anything Karcher below the K4 as they use plastic impellers which are prone to failure.
When compared like for like, the Nilfisk's are better value for money but as with most things it's personal preference.
I prefer Karcher, purely because the warranty is replacement rather than repair and I have a Karcher centre literally 5 minutes from my house should it suffer any issues.
I had a K3 for about 18 months and have just replaced it with a K5 Premium EcoLogic.