I would say that Jo's right - OW is a league on top of Deloitte. The salary is much higher, it's a much better brand on your CV and you'll learn a lot faster. OW is more comparable to the likes of Marakon and Monitor than Deloitte.
From what I've seen firsthand of OW they do work hard and their approach is quite dull, but I doubt they're the worst in the business in that.
Entry-level salaries will be roughly comparable (others will know better, I'd guess 30k for Deloitte and 35k for OW), but after 3 years OW salary could be almost twice the Deloitte one.