I don't know the answer, to be honest. Rationally one might say the 3-year LLB grad must know more than the GDL student or 2-year LLB grad. Is that necessarily true? I don't think it is. Really, a Law degree is transient and fast-paced. You learn so much and you forget so much. When it comes to TCs, someone who last studied criminal law in their first year of their 3-year LLB might have a lesser command of criminal law than the GDL who studied it that year. If you apply that argument to other modules, then it starts to count. Also having studied more modules is not necessarily an advantage. If two people got 72% in commercial law and both are applying for TC in commercial practice, what does it matter that one studied the law of evidence and the other one did not?
However, and this is most anecdotal so bear that in mind, researching firms I have found most people who do GDL and succeed to practice were Oxbridge undergrads, whereas there are a lot fewer people who have done GDL and are non-Oxbridge who succeeded to practice. I am not sure what you can take from this! Perhaps it means Oxbridge grads are more likely to get into firms and they are also more likely to be polymathic (hence studying another subject at undergrad), whereas non-Oxbridge grads are more likely to go for the pure LLB route; or perhaps it means GDL students who are not Oxbridge calibre are at a disadvantage to other applicants who are also non-Oxbridge, and that's why they rarely show up. It could mean a multitude of things.