Just different, similar but different to the degree you might want both.
HD800 airy, delicate, uber-spacious with incredible amount of details, T1 bold, perhaps more aggressive, closed in and intimate, better choice for less than stellar rock recordings.
Microscope vs magnifying glass. Technically HD800 is clearly superior (I do no wrong), T1 could be more emotional. That's just my personal perception.
You nailed it! That's exactly how I perceive them as well.