Hello,
When you say connect a Hub to a Router, are you putting the Virgin Media Hub in to "Modem Mode" and then connecting that to your own Router?
If so, I would say you should opt for a Cat 6 Ethernet cable. Also, if the Hub is in "Modem Mode", you need to connect one end of the Ethernet Cable to Port 4 of the Virgin Media Hub as this is the only one that will work in Modem Mode.
The length of the Ethernet cable doesn't really matter unless you are talking a really long cable!
Hope this helps.
🙂