I'll try to give you my perspective as a parent whose daughter played her first year of club VB (U15) at The St. James. I obviously cannot give you many specifics on the TSJ boys program ... and I've got no knowledge of your alternate option.
One of the TSJ boys coaches is now the girls' varsity head coach at Flint Hill (promoted from JV coach, I think). Many folks may argue that Flint Hill is the best girls' program in VA. This doesn't mean that the coach will be your son's coach this year, but that coach will at least be "in the room." Can't be bad, right?
The TSJ boys program also has decent/strong ties to the men's VB program at George Mason University. I think there are only 30 or so Division I men's VB programs in the country and GMU is one of the few on the East Coast. The NCAA men's championships were held in Fairfax earlier this summer.
Many of the boys and girls teams practiced during the same time slots, so there was plenty of interaction (including scrimmaging) between teams and coaches regardless of gender.
Depending on your son's goals, I cannot identify an immediate reason to rule out TSJ. You may have to try it one year to see if there is a fit and be willing to move on next year if needed. We made the choice to pursue club VB for our DD to better position her to make her high school's varsity team this year, after earning a spot on JV as a 9th grader. Although her school's entire VB program has been weak recently, the mission was accomplished (and the varsity team has already won more games than it had in the last two years combined). We may not be returning to TSJ this year given DD's "upgraded" VB goals now that we have a better understanding of the club landscape on the girls side. However, that does not mean we view the season at TSJ as a poor or bad experience.