They are different, no doubt. The T1.2 is a vast improvement over the first version. Against the HD800, they are two very different flavors. The HD800 is still much more spacious sounding and transparent, but an argument can definitely be made for the T1.2 in terms of musicality and "fun" factor. There is always a risk involved with the HD800 that you will be annoyed by the treble... it's very much a headphone I would recommend that you try before you buy. With the right amp or mods, there is some improvement to be had in this area, but I've always preferred a well amped stock HD800 to a modded one.
The T1.2 is much safer in terms of treble now and adds a bit of extra warmth for good measure. I still don't find it to be as detailed, spacious or clear as the HD800, but it will be a lot easier to enjoy, I think, for many people. I think people who are looking at both of these should also be looking at the Enigma Acoustics Dharma D-1000 and the Mr. Speakers Ether, which split some of the differences between these two well known flagships.
To my ears, the Dharma takes the massive soundstage of the HD800 and combines it with the musicality of the T1.2. The Ether gets all the way up to the HD800 in terms of clarity, but might be a tad less musical relative to the T1.2, depending on your preferences. (I, personally, don't find it any less musical, but YMMV.)
Together, I think the 4 of them provide a nice spread in terms of options for a neutral, clear and spacious flagship in that price range. It's nice that there are a couple of options in the middle now to split the difference.
