I bought mine form Bamboo Gaming. It's the cheapest place I could find online, but you have to wait 2 weeks for them to ship it from China. If you want it quickly, consoleup.com has them for £22 + Delivery
DS all the way. So many great games for the system, better battery life, cheaper too, and if your into movies/homebrew it's not too difficult or expensive to do on DS either. Not to mention a large back catalouge of GBA games to play too
btw, although a PSP with a Sony 3.51 firmware can't be downgraded yet, there is a custom 3.51 firmware out (several, from the looks of it) by Team M33, a bunch of russians continuing DaX's work it seems.