Nant Yr Arian is like Coed y brenin lite. It has a lovely quick loop with three or four nice singletrack descents, and rewards someone with a long day as well. The riding is mostly like the above – close and tight treelined singletrack.
Cwm Rhaeadr is much further south, near Brechffa. It’s one long (probably 15 mins descending) drop with a slow fireroad climb back up like a rollercoaster. Lots of twisty, turning, high berms, more like half way between singletrack and a 4x track and a dh track.
If you have a day to spend somewhere, I would go to Coed Y Brenin and do the beast (or even the MBR route). It’s worth the drive.
If I had two days to spend at welsh centres, I would drive early to CYB, do the MBR route, drive south to Mach and do a few loops of climachx (the longer you have the more singletrack you can do, but it’s all pretty weak in comparison to the final descent). I would then drive south, spend a morning doing cwm rhaeadr and an afternoon at brechffa. But you will be pooped.