Well the choice is yours really get a PSP or a Xbox 360.
The only problem is that Xbox 360's are selling like mad right now and are going for 400 GBP + each.
As for the Sony Playstation Portable its going for around 100-200GBP a lot cheaper
I have a PSP myself and am addicted like mad to it. I don't find time to do anything else but play on my PSP. When I'm waiting for a bus, waiting for my eBay listings to end or in the toilet the PSP is fantastic.
As for the Xbox 360 I have a couple of them but rather then playing them, like most other people I'm selling them why? to make some quick money why else.
I believe for the moment it is better to get a PSP cheaply and get addicted to it like me and if you still want a Xbox 360 wait.... a couple of months for it to be back in stock then get it at its retail price as for if you have a Xbox 360 now the only sane thing I can think about doing with it is selling it on eBay.
Hope this helps in making your choice between a PSP and an Xbox 360.