Two of my favourite cities
Prepare for an essay.
Both are similar in many ways. For a start they are both very student orientated, quite young and vibrant. Newcastle has quite a reputation for being the party capital of the country (and even Europe) but it has its laid back, cultured side with some great theatres, museums and galleries (particularly since the gentrification of the quayside of capital of culture bid). Cardiff too has a good cubbing secen (so I am told) and also went for the capital of culture bid for 2008. So it's also seen a great deal of investment and regeneration. With Newcastle it's largely the quayside, with Cardiff it's the bay area (great for recreation, good museums and restaurants). Both also act as a great base for exploring the wider region. Both have great countryside on their doorstep although Newcastle (with Northumberland and County Durham's castles and World Heritage Site) take it for me. That said, Wales isn't short of castles either.
Another similarity is that both are quite compact city centres, which really works in their favour. They are certainly two difference cities though and there are certain contrasts. The most striking include: -
Both have a great music scene but very different ones. Newcastle gets its fair share of major bands and artists, Cardiff does but to a slightly lesser extent. Newcastle has a great local scene (more unisgned bands than any other city outside London), there are also some great clubs for all tastes (rock, jazz, hip hop). But Cardiff certainly has a scene all of its own. I think its strong Welsh identity is seen most of all in its art as well as its music. Both are very interesting and exciting scenes.
Architecture. I don't really know how important this is to you. Newcastle is one of the most beautiful city centres in the country in my opinion. It has plenty of grand Neo-classical architecutre (and modern architecture eg. Sage). Cardiff seems to be a lot younger and its architecture reflects this. When I first visited Cardiff my first impression was that it was quite sterile. My first impression is wrong. But Cardiff. The uni still has some grand buildings though, based in the old civic buildings and they include some white, neo-classical buildings. Both have castles, but Newcastle has the proper castle
Weather. Cardiff is warmer but has far more rain; Newcastle is milder but drier.
But both cities have it all, clubs, bars, restaurants, shopping, theatre, culture. Both also offer a reasonable cost of living, again helping their status as "student cities".
Newcastle probably does better graduate prospects wise (graduates in employment, above average graduate salary) but I wouldn't let this influence you too much. You can still have great career prospects from Cardiff as it too is a fine uni. It's up to you to put the effort in. At the end of the day you're the one who makes your own career prospects, not the university. As for the politics department, I don't know a great deal about the courses and areas they cover. Both are solid departments though. Do you have any preference course-wise?
I may be back this evening to give a more detailed comparison between the two, if you're interested in anything specific then let me know. I'm also moving it to the politics forum.