There’s a bit of most things at Cwmcarn depending on what you like.
The blue is short and not wildly exciting – it’s alright if you finish something else and want a quick 20 min extra lap.
Cafell and Twrch are xc loops – but they’re not like other xc loops really. Both have decent elevation, some fast and bumpy descents and hard climbs etc.
I’d say Cafell is harder on your fitness with climbing, Twrch is harder on technical climbing.
There are a couple of runs classed as downhill – 1 goes off the top of Cafell – and is pretty fast and flowy. The other goes from the top of a hill that you can get to from about halfway round the Twrch where you go up a short / steep concrete ramp.
If you like steep / tech off piste the you want the Risca side of the hill – you can either go to that concrete ramp I mentioned on the Twrch or you can pedal up the tarmac Forest Drive and look out for the off piste trails. Most of them are on Trailforks.