I have used both. YMMV but my opinion is that once you are going to use a remote that requires two hands, you might as well use the one with more resolution. I just feel as thought the TPMC-8X gives so much more ability to create a beautiful GUI by using 800 x 600 instead opf 640 x 480. Someone is going to tell me I'm wrong amd also point out how all the hard buttons on the 6X effectively give you more screen real estate but I'm fully aware of that and only stating my *preference*. When I hand someone a 8X I am able to give them a pretty powerful whole house control GUI that I just don't have space to create on the 6X using our preferred GUI.
There is no doubt that the 6X is a bit better for channel surfing, but for that I'd actually go with the 3X. When budget allows the perfect combination is a big wired panel for full house control and a 3X for TV. BUT...the amazing thing about the 3X is that it can even work quite well for basic house control due to the 2-way feedback.