As someone who's lived in Cardiff and Bristol I'd like to just add that if you want to see decay, despair and abject poverty then have a wander round most of Bristol City!
There are scarier people, places and sights in that city than I ever saw in Cardiff! Beggars on the street everywhere, ghettos, drug related shootings an everyday occurence..Bristol's an overrated, overpriced dump IMHO.
Cardiff's taken enormous strides forward in the last ten years and is now as cosmopolitan as Bristol thinks it is.