At this point in your career, I would tend to recommend that you go for the internship that has the most in common with the job that you ultimately want. Some reasons include
- You don’t have to explain why you’re “changing roles”
- There can be a stigma associated with going from software development to project management or vice versa
- The hardest part about getting a career started is getting that first bit of “relevant experience” because lots of places don’t want to hire someone that’s never done the job before at all
Also, I would disagree with the adage “the best car salesman is a mechanic” because they’re different types of people. A good salesperson is a people person. Yes, they have to know the product to a certain extent to sell it, but what you need to know to be even a decent mechanic is way more than you need to know to be a good car salesperson. The biggest factors in being a good salesperson, of any product, is being personable and how to help people be comfortable with their decision to fork over their hard-earned money.
Similarly, project management is very much about how to deal with people and coordinating their different personalities. Sure, if you take a software development job (or internship) you’ll learn about the specific challenges that software people face … but you’ll learn that too if you’re a project manager and listen to the people you’re working with.
Finally, I think the biggest reason why I think you should take the internship that is closest to the job that you ultimately want is that you might try out IT project management and learn that you hate it and want to do something else with your career. It’s much better to learn that you hate something on an internship that is only going to last several weeks to a few months rather than after you’ve taken your first “real job”
Whatever you decide, just know that no decision you make is permanent and even outright mistakes can be handled and learned from. If you see everything as a learning experience, then perhaps the decision will seem less daunting and easier to make.