Why not just list both and explain the first is unweighted and the second is weighted, especially for an e-mail rather than a form that only permits one number?
Anecdotally, my impression from admissions folks is that they find the weighted GPA arms race pretty tiring. It used to be that only AP classes had weighted grades and there weren't that many AP subjects. Now, not only is there an AP for just about every subject (many of which don't even have a non-AP course option), but in many cases university departments don't actually consider them equivalent to their basic courses. You can get credit that allows you to skip an elective, but not place out of their required courses, which makes it hard for Admissions to view it with the excitement they used to have for an AP course in the past. Plus, some high schools are not offering AP classes and either not offering weighted grades or just calling any course they consider "advanced" to be a weighted grade-eligible course. That's why you end up with a fairly wide range of GPAs from students at different high schools who are really equivalent academic performers.
So, what admissions people will tell you is that they look at the true (unweighted) GPA to see how successful you were in your courses (as opposed to weighted, where you may be using several weighted grades to bring up a lower grade) and they look at the courses considered "advanced" by the school (whether weighted in grades or not) to see if you challenged yourself to take the most difficult curriculum possible at the school.
All of that, of course, means it's really hard for a soccer coach to just look at a single number now and know whether you have the academic chops to be competitive for admissions. That's why both is probably ideal (or telling them if your school doesn't offer weighted grades or if weighted courses only start in junior year etc) and if you can tell them what the weighted GPA is out of, might as well throw that in there too.