I've lived in Cardiff all my life and it is a great place to live on the whole. It is really compact so you can get around easily, even without a car, it has a lot of facilities, some amazing buildings/architecture, great shopping, it is really close to some amazing countryside, the coast etc, but it has some issues too (maybe these won't bother you):
1. As it is the biggest city in the area it is a magnet for people from miles around. Coupled with the fact that it is very compact this means that it gets VERY busy on weekends.
2. The stadium. They close ALL the roads in the city centre when something is on in the stadium, close the bus station/train station, reroute all the public transport and turn the entire city into a giant coach park. Most of the shops close and the city centre is a virtual no-go area filled with rubbish, drunks and police. Everyone who visits the stadium thinks it is great but for the people who live here it is a pain. The stadium holds 75,000 people, so on an event day about 100,000 extra people are in town. The population of Cardiff is about 300,000. That's 1/3 the population of the city! That's like an extra 3 million people deciding to go to London one day! I don't leave the house on match days!
3. Property prices are going sky high.
4. When I was in uni I used to go out to a club about 3 times a week, but now only a couple of years later I don't feel safe in the city centre at night. The night-time seems to be getting "heavy" in the city centre. Most of the bars are all along one street so there is a high concentration of people, alcohol and inevitably...trouble in a very small place. There are a zillion police, riot vans, police horses etc every fri/sat/sun night.
As for Swansea, Swansea is an absolutely amazing place to be in the summer. You can only imagine how stunning the scenery and beaches are on the gower penninsular (area of outstanding natural beauty and a national park); however be warned that Swansea is grey and depressing in winter. The wind and rain NEVER end!
Drop me a line if there's anything you want to know about Cardiff! It's not all bad - honest! Good luck with the move!