The K712 is a warmer sounding and is smoother and darker with fuller mids. It's also grainier and more dynamically compressed.
The T90 is lighter and airier sounding with clear and forward treble. It's more detailed and resolving than the K712 and has greater dynamics. The soundstage is good on both, but the T90 has an edge in terms of more clear and effortless separation. The T90 also has larger sounding instruments and a taller sounding soundstage.
The only word of caution on the T90 is that it might be too airy, and the treble too forward. It would depend on what genres your planning on using them for though. The K712 is less likely to be polarizing in any area.
Comfort is great on both, with a slight edge to the K712 . Ear cup depth is pretty similar, but the K712 earcups have a bigger diameter. K712 is a tad lighter, but I can wear both for hours with no issues.