I have both. Actually bought the on2 because headband on the ESW9 broke and ATH quoted me over €100 for replacement parts.
I love them both in different way, ESW9 is little bit more relaxed with nice mids (as it is ATH).
On2 little bit more balanced to my ears.
As Dsnuts said, you are in for the treat. I mean, I havent met anyone who would not like it On2, they are one of the best portables I've heard.
I haven't heard the DT1350 but I used to own the T51p. I liked them, but I think On2 are better all around.
Just to add, Thinksound CS is brilliant. One of my cables broke (you get 2 with On2) and I have emailed them about it, got reply in less than 6 hours that cable is on the way to me. 5 days and I've received replacement cable in EU, no questions asked.