First, do check the reviews by other users on the Cardiff site. The Novotel seems to be going through rather a renaissance at the moment following an awful patch. The Royal is fairly newly refurbished and re-opened as a hotel and seems to be OK but has some down points which the user reviews will set out.
For my own opinion, yes the Royal is slap bang in the centre, but I do not know what the noise situation might be like - the area of St Mary Street is very lively with clubs and bars (the Royal itself is over one of them) and sits at a busy interchange.
The Novotel is in a quieter area but still only about 10 minutes walk from the city centre.
If I had to plump for one I would probably go for the Novotel.
I did have a look at the reviews, especially the ones for the Royal. I'm trying to do a bit of research for my daughter. That was one thing that was concerning me about the Royal - the club scene and the noise.
It's for a Sunday night and I would assume that is a quieter night. She just wants a place to sleep while doing the sights.
Is the Novotel located in an area that would be safe to walk back to just before dark?
In that case the Royal might be a better bet, it will be quieter on a Sunday night. The Novotel will be safe enough but for your (and your daughter's) peace of mind the central 'busy' location of the Royal will be better suited. I take it she is travelling by train? The Royal is just a few minutes walk from the central station. For the Novotel she would be better off getting a taxi. Perhaps when booking you could ask for a room on one of the higher floors but not the top floor as I believe those rooms are smaller.
Thanks again Jim. Will let you know how she gets on.