Are there any other students who are going to take middle school math next year? If there are, you might be able to carpool. If it was my child, I'd lean toward the middle school class even if it was inconvenient simply because (in our school district) school staff wouldn't have any wiggle room to question whether or not my child had completed the course successfully once it's over - our staff tend to question mastery through outside curriculums, even when they're through a program such as EPGY.
Is it only a one-grade level bump? And what level of math is it a bump up to - first year middle school math or pre-algebra or higher? If it's just a one-year bump and especially if it's only a bump up to first year middle school math, I'd suspect there would be other children in the school who could also handle the coursework... and it would make sense to try to come up with a way to handle it as a small group rather than just one student going to the middle school or working individually (?)... just curious!
Good luck with your decision!
polarbear
