Generally, it is much more difficult to secure a graduate position directly rather than through an internship (this applies to big 4, ib's, law firms and most large corporates who have vacation programs).
For example, UBS last year took in 14 interns in their ib division, 6 of whom were made graduate offers, but they only made 2 graduate offers to fresh applicants this year (partly due to market conditions no doubt).
4 of the 6 top tier law firms didn't even have a formal graduate application process this year, as their graduate intake starting in 2009 was almost entirely filled from summer clerks, recruiting on an ad hoc basis to fill the very few remaining spots.
In relation to big 4's, it generally is about 3x as difficult (in terms of applicants to offers ratio) in graduate recruiting as internship recruiting. This makes sense since with internship apps you're (generally) only competing against penultimates, yet come grad application time you're competing with final years, those who have graduated in the last 18 months (cutoff for eligibility to most grad programs) and often even masters students.
