Sorry - never having been to either, but have been to many other UK circuits in my car & on experience days. Of the two, Oulton looks more exciting. Changes of elevation & such, whereas Rockingham is 'just' an oval with an infield... I just mean Oulton will feel like a more exhilarating experience (instructors dependent of course...).
Spend the extra too - you'll regret not driving those cars!