First of all, congratulations
@LEL and
@thirdtimelucky on securing vacation schemes! It is an exciting position to be in where you are about to experience the inner workings of a commercial law firm for the first time
I have seen some great suggestions on this topic and I think that the first thing to consider is whether you will have the opportunity to explore multiple seats throughout your time on the scheme or if you only get to explore one. If you have the opportunity to sit in multiple departments, then I would definitely take the recommendations that some users have given to sit in contrasting departments - perhaps one corporate seat and one disputes or advisory seat.
When making this decision myself, for example, I knew that I was really interested in trying competition/antitrust because a lot of the regulatory side of competition law is related to politics and international relations which I really enjoyed studying at uni. At the same time, I did not know enough about what being a competition lawyer entailed so if I had only had the choice of a single seat, I probably would have gone for one of the firm's bigger departments - corporate or litigation. At this firm, I ended up sitting in two seats - Private Equity and Competition, so I got the best of both worlds. Competition proved to be as interesting as I thought it was, partly because the team had its own clients and was not purely a support team, which may have also impacted my decision making.
I think personally that you cannot go wrong with a corporate seat at any commercial firm as you will gain exposure to a lot of the clients and work that drives the firm as a business! You will often have the chance to liaise with members from supporting teams and you will likely sit alongside plenty of other candidates for your seat to be well-structured enough that the firm is able to accurately assess you.
If you are really into disputes, then by all means choose that as your single seat and if you are interested in another more specialised area then I would be more inclined to choose it if it is an area where the firm specialises. As another user advised, it is good to get a sense of what the firm's specialisms and compulsory seats are so that you can have an accurate understanding of what it would be like to work at the firm in question when you come to choosing whether to train there