Cardiff for me, longer recent history at higher levels and 2 recent 'major' cup finals.
Swansea may well be currently in the Premier League but they were only promoted through the play-offs, with a little luck, like Swansea, Cardiff could have been in the Premier League in 2010.
Cardiff City fans have also been unfortunate to not have seen their team qualify for Europe. They were beaten by Portsmouth in the FA Cup Final in 2008 and by Liverpool in the Carling Cup Final in 2012. Both Portsmouth and Liverpool had not qualified for Europe via their own league position.
Portsmouth were the 1st club since Everton in 1994 to win the FA Cup and not finish in the top 4 of the Premier League meaning Cardiff didn't qualify as a losing finalist like most other runners-up! Defeat to Liverpool in the 2012 Carling Cup was both undeserved and unusual because Liverpool have only finished outside the top 5 on 4 occasions in last 18 years!!