I start an Assistant Professor position in August (in USA). Is it better to hire a Phd in August or January?
Often, your first semester in a faculty position flies by...especially if you are teaching (which you do not mention). You often meet with HR and administrators, learn where everything is on campus, and settle into your new home/hometown (I've started as a new faculty member at two different universities in my career). Consider that you might not have a lot of time to supervise and manage people during your first semester, which will waste your start-up funds. Waiting to hire the candidate in January will give you time to set up your program of research and have a full list of work for the candidate to work on when they start. Just a thought. Congrats on the job!