we have been to cardiff and the last 2 years at murrayfield, and i have got to say cardiff in our opinion was by far the better experience, however the last time we were there it was May so a bit warmer so able to walk around cardiff and find places to eat and drink not tied to paying top dollar in the stadium.
We stayed outside of cardiff on a caravan park which was full of rugby fans, unfortunately this isn't open in February so probably quite a lot of families who find the caravan option a lot cheaper will not be able to go.
If the event had been as in previous years on the may bank holiday we would have been there but the expensive accomodation, the unpredicatable weather, and the posibility of having to travel back to yorkshire late on sunday and getting up for work monday means this year we will be watching on tv