Depends a lot on how old your son is and what kind of games does he like. We have both the DS (my 7 year old daughter's) and the PSP (hubby's), as a general rule I wouldn't buy the PSP for a child under 12 years old, and this is directly connected to the kind of games available for the two consoles (Nintendo's are more kid friendly). This said we are looking into getting the new PSP when we go to WDW in November with the plan of recycling the "old" version for my daughter since she does enjoy playing some PSP games (Shrek the 3rd, LocoRoco, Talkman etc). She is way above her age when it comes to games & computers though.
So pros for the DS:
Superior battery life
More arcadey games
More simulation games (nice for kids, ie Nintendogs, Cooking Mama, Animal Crossing, my daughter loves them all)
Probably more "interactive" thanks to the touch screen that appeals especially to kids
Pros for the PSP:
Technologically superior (much better graphics)
Lots of cool features for viewing photos (you can even get an add-on camera) and listening to music etc
Internet browser, music player etc included
Frequent updates from Sony to get even more nice features
Interacts with the PS3 if you have it
More appealing to older kids and adults (in my opinion) who like adventure/role playing/driving etc games
Price wise for me the DS is overpriced for its technological features. The PSP is one of the coolest things on earth (always in my opinion), the game selection IS limited but then again how many games do you really want to buy for a portable console? The battery life is an issue with the PSP, but I would not let my child play more than 4-5 hours per day anyway...
Regarding buying in the US it is definitely worth it for either console (although I'm going by the italian prices but I guess they are pretty similar in the UK) especially at the moment with the current dollar rate. PSP UMD movies will be region coded but if you are interested in them, you could buy a few while you are over there (the selection should be much wider too) to start with, and you can always buy new ones via internet (that usually end up costing the same, if not less, than buying them locally).
Just my opinion people, I had no intention to offend any DS OR PSP owners and do not want to pick up a fight with anyone (like so often seems to happen in forums when ppl are comparing any video game consoles)
Jo
PS I personally don't find the (old model) PSP any more uncomfortable to play with than the DS