The first two Ultramixes are not very good at all, for DDR standards. I've not played UM3... so it might be better.
I'm more an Extreme JP / Max 2 player, myself... although my true calling for dancing sims is In the Groove.
1) They can actually make HARD charts. Judging from what I've seen of Supernova, Konami's solution to increase the difficulty is to spit out a bunch of idioticly hard (I'm looking at you, Fascination MaxX) charts that don't really go to anything, make a bunch of easy charts (I'm looking at you, Mugen) and slap a "10" on them, or make more Oni courses, which I don't like because forced 1x is completely stupid. I don't even play the Max songs on 1x. And a boatload of 8s and 7s don't help, because perfect attack is NOT fun. Also, DDR has a tendency to "dumb down" stepcharts and give them rhythms that make no sense (eighth note stream syndrome). ITG is guilty of this at times, too, but it's not nearly as bad in my opinion.
2) I like ITG's noteskins much better. DDR's default skin, to me, is no better than flat (forced default noteskin is what kills me on Extra Stage). So I have to play Solo, which gets bad when the background switches to blue or red and all the arrows blend into the background.
3) ITG has a more diverse and enjoyable selection of music. DDR strikes me as having too much J-Pop and techno. I like techno, but come on. Naoki made like 80% of the songs on DDR. ITG has some songs that aren't really thought of as "dance sim" songs that I still like; sythesized instrumental music (Anything by Dust Devil, and Tell), Happy Hardcore (Smiley -- while I admit DDR also has some of this, I like Smiley better for some reason -- and yes, I know that Smiley is the same person as like 6 of the artists on ITG), and a few genres I can't describe that I like (Anubis/Banzai, most songs by Inspector K... etc. ... Yes, I know those two are BOTH the same person as Smiley).
4) HANDS. Hand charts are fun. Rolls are fun, too, when used right.
5) Built-in custom step-chart support. Not sure if DDR has this, but ITG's USB support allows you to load your own custom simfiles on an arcade machine and play them.
So... ya. That's my personal dance sim experience.