It's a real tough decision on this one.
I know we are talking about the DS here, but before I get into it, I have to say that Nintendo's pre-show and display at E3 was absolutely amazing. Especially compared to last years show.
I actually think Penny-Arcade's comic describes it perfectly, that fact being is both could have shined in a lot more areas. But we still have much more to see.
I mean, it's quite understandable that the DS double screen (see: Game and Watch) could be confusing and poorly demonstrated. On the contrary, however, Nintendo hasn't shown much utilization with it yet; I hope they will be displaying and sharing ideas with it soon.
As for the PSP? Well, aside from the projected extremely short battery life and what we have seen so far with the games (In my opinion, I think in the gaming category, both companies took a hold to this aspect and seem to be doing excellent, balancing this issue out), it seems to be looking great in many dimensions, ones that I shouldn't have to point out.
So it seems like they balance out a bit so far right? Well, that's because we don't have as much information as we should have had at this years E3. Unfortunately, they both could have demonstrated performance a lot in more areas. As I look at the whole picture now, it seems to come to a contest of image and who will court more 3rd party offerings (which Nintendo as failed to do in the past).
I'm quite excited to see how this unfolds.