Both have their merits, so it's hard to choose. But if it was based on value alone, you cannot beat a Dell - especially with the discount, cashback & cheaper RAM upgrade....as detailed below:
Use Dell 10% voucher 22HGWHCJM$NW1R at checkout (works on XPS systems only) - your order needs to be a minimum of £599 (excl VAT & del), which shouldn't be a problem here I think! EXPIRES 5/03/08.
In addition, go through Quidco click here to get 5% cashback (on order value excl VAT & del) - that's £30 on a £600 order for example.
I'm sure I can cut it down a little further by getting the RAM upgrade to 4Gb from elsewhere (e.g. eBuyer), as Dell's ones are too dear. If this reduces the order below £599 (i.e. misses 10% discount), then don't do this. G