Personally, I don't think you should factor in the complexity of tasks or business need. This is because (1) it's impossible to know what specific tasks you will get from the people in your department, and (2) the busyness of a department does not really reflect how likely you are to convert the scheme, your workload, or your experience in that team.
I'd factor in almost everything based on your personal interest. The only other factors I'd think about include any experiences you have had meeting people in that practice area, or any comments you've heard from people at the firm about what that practice area is like. But I appreciate that is likely to be minimal.
The reason personal interest is the most important thing is it'll make a huge difference to how you show up, the kind of questions you ask, and how quickly you pick up the work. If you're not sure what you're interested in yet, I'd start by reading about the practice areas on offer and making a decision based on this. You could also find the trainees/associates on LinkedIn and see if you can get a vibe about what the team is like (but this is a very minimal view on what it's actually going to be like).
