I'm not going to go into detail, but the Zio has a nicer screen than the Ascend, the Ascend is more responsive, the Ascend only has 2.5 mm jack as you mentioned, but it does come with headphones that will work. The Zio comes with 1 GB Micro SD and the Ascend comes with 2 GB Micro SD. The Zio's processor is better, but the Ascend seems to perform better so far. There is an issue with the Zio's touchscreen. I had some trouble with it on Android 2.1, too. I like the Ascend better than the Zio so far. I've used the Zio in person, but not the Ascend. I did hear that the Ascend's battery life is better. According to CNET, the call quality on the Ascend is bad.
I know this is kind of messy but I typed it up in a hurry. I hope I helped you out somewhat.
-Avenue