Unless you want to study something in which Oxford has an obvious and unrivalled specialty (Philosophy of Law, for example) then this is surely a no-brainer, and the more so since you've already had the Oxford experience.
I vaguely feel as well that 'Oxford + UCL' is if anything more impressive than 'Oxford + Oxford', as there's sometimes a suspicion that it's easier to stay put and be advantaged in admissions only as the 'known quantity' candidate. And it is surely that when you can as well put "Research council funded" on the CV.