Those are loaded type question unless you are not a Textures fan and do a strict comparison. It's like asking what Circus Maximus is doing that Dream Theater hasn't already done better so why enjoy Circus Maximus. That same comparison can be done across hundreds of bands of any genres.
As for your question though, I do not find Textures & Disperse that similar except in the mellower tunes. However, I will hit a few key points for me:
*There is a huge difference vocally. When Textures uses clean vocals on their mellower tracks, then I enjoy them. When they use the dominant rough style, I hit the eject button in a hurry. I cannot even pay attention to the music because of the distraction of the vocals. And for the record, I don't mind rough vocals pending on the style employed.
*The Disperse guitarist is in another league. Once again, the mellow bits between the two bands that sound similar are a push. However, when it comes to out and out shredding along with odd rhythms & note choices, it's no contest for me.
*I find the compositions/songs much more interesting in Disperse. There is no immediate hook so I really have to pay attention to what is going on and that draws me in even further. When you expect him to shred, he pulls back and goes tasteful. When you think it's too mellow, all hell breaks loose. Obviously, that's just personal preference and you (along with others) may feel it the exact reverse.
Don't compare the mellow stuff. Compare the normal stuff and there is a world of difference other than guitar tone:
http://www.youtube.com/watch?v=FgJQMPmmi5U
http://www.youtube.com/watch?v=ynnsXs_hiSM
I would rather punch myself in the nuts with a hammer than put that Textures track on repeat.