I'm after some advice about runing network cables in my house.  I have the opportunity during our current renovations to run a cable up into the roof space of our 2-storey house, the stud is around 3.1m each floor so I need the cable to be reasonably long to cover the distance, the router isn't really that close to where the cable would go up the wall so I suspect I'd need another 5m at least so a 15-20m cable.  I have had a look on ascent.co.nz and there are a few options.  I would like to know if :

1.  a cable that long will still be useful or will the degradation in speed/signal mean that WiFi would be just as good?

2.  Should I buy a reel of cable and then get Ethernet boxes at either end, or buy a readymade cable with RJ45 connectors?

3.  I want the best so Cat 6 or Cat 6a or something else?

Thanks in advance

Andy