How to delay a potential advisor without burning bridge?
There shouldn't be any guilt involved since you don't seem to have made any commitment. I think there are some important points to be made in a note totem.

Thank you for your interest and support
I'm feeling a bit overwhelmed right now and need a bit of time to explore my options and further directions.
I'll be more than happy to meet with you when the term starts for additional advice.

If he has already said that he wants you to be his student then it is a bit tricky, of course.

I'm delighted that you would consider me as your student, but ... (point 2 above)

If he has not been explicit about taking you on, then don't be presumptuous, of course.

I'm currently trying to decide on an advisor, but (point 2, again)

Then, find out some more about this professor and the other. If they are, in fact, not tenured but about to be evaluated then you need to know that. It can get in the way. Look at a few papers by each potential advisor. Talk to a few other grad students if possible. The department might be able to put you in touch with a few.
Let the wording be your own, but you haven't walked through the door and you aren't closing the door.
If you want to prime the pump a bit, ask for recommendations for a paper or two that you might explore now to help you make a decision.