Microsoft really dropped the ball when it comes to simplifying and clarifying the differences between various Windows 8 devices, so you're going to have to be really careful in your research.
There're currently a few Windows RT/8 tablets released/slated for release:
- Windows RT tablets running on ARM processors
- Windows 8 tablets running on Atom processors
- Windows 8 tablets running on ULV i3/i5 processors
- Windows 8 laptop/tablet hybrids running on i3/i5/i7 processors
Of these, I'd expect only (3) and (4) to really be able to handle 10-bit software decoding. The Iconia W500 runs on an AMD C-50 (AMD's category equivalent of Intel's Atom, minor quibbling over performance differences aside), so its inability to handle high-bitrate 10-bit video is not surprising. Ultimately, you still need relatively high hardware specs to get 1080p 10-bit decoding.
The video chipset is not going to matter, at least until we actually get hardware 10 bit decoding support. Get something with an Intel i3/i5, since I haven't heard of any AMD-powered tablets headed our way yet, aside from
the Fujitsu Z-60 ones (although keep in mind the Z-60 is still in the same class as Atom).