Have both the PowerDVD 17 Ultra and the latest DVDFab Player 5 Ultra. Here is what I can tell, in regards to UHD Playback (might not suitable for other features, though):
1. Specs wise, it looks like PowerDVD's requirements on both hardware and software are way more sophisticated than the Fab Player, I guess one, if not tech sensitive enough, can feel very hard to figure out how to get the UHD playback feature to work flawlessly; the Fab Player, on the contrary, only requires a GPU that is capable of H.265 decoding;
2. By UHD, if you meant to include the 4K Ultra HD Blu-rays, then I think the PowerDVD has one legitimate advantage, which is, it does not require a 3rd party decryption software to play back the 4K Ultra HD Blu-ray discs; however, with the Fab Player, I can only play back the already decrypted 4K Ultra HD contents, meaning, you will have to rely on a 3rd party decryption tool;
3. All that said, from the pure perspective of playback quality itself, I don't think there is any difference between the two;
So, my advice is, if you are a tech savvy and don't bother to dive deep into all those jargon and terminologies, then head for the PowerDVD, after all, it's the only legal solution authorized by the BDA. If you are a simple guy who wants everything to be as simple as the way it is, then the Fab Player is probably the better shot for you. But bear in mind, it does not play back the protected 4K UHD discs, like mentioned earlier, you will need a decryption tool first.
Edit: Does AnyDVD now support to decrypt 4K Ultra HD Blu-ray discs? I thought the only one available, at least for now, is DeUHD, the Russian vendor?