Great laptop if you can get one at a decent price, which is the issue here.
You're paying 400 pounds more for a laptop with a thinner profile and the same specs, it really isn't worth it.
A 25% heavier laptop is definitely worth it for a saving of 400 pounds, I mean it's still relatively slim, and you're going to be using it on a desk or carrying it on your back most of the time anyway.
It's 500g of weight, but if you really want to pay more for the XPS 15, I can't stop you.