Just get both eventually... Both are becoming equals in terms of quality. Like lately, I haven't been too impressed with the DS. I mean before I was, but since I got it and got some of the games that make it great, the newer announcements haven't really caught me eyes.
While I've had a PSP since firmware 2.0 was brand new, I got bored with it fast until I finally got it fully unlocked and learned how to convert my PS1 games to PSP. The PSP is also getting a great line up. Companies like Square and Konami are putting a lot of time and effort into it now. Not to mention, all the advanced features. You can easily get a used PSP for cheaper than a DS aswell.
Either way, you need both if you want to continue your KH experience.