Paying for ad removal simply removes the adverts.
Premium Support means you get support faster. As far as I know there is no guarantee on response time. It's just that those who pay go to the head of the queue and are likely to get more heavyweight customising of their forum done for them. Try posting a message on the Premium Support forum to discover the rates.
EDIT:
It's also worth saying that if it's something that only one of the Nabble Team can sort out, rather than general advice and guidance on use of the software, and it is reported over a weekend then it can take a few days. Nabble is a small operation and support is largely a one man operation and the guy does take more time off at weekends.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.