We are doing it on our Epcot day as this was announced after we made our plans, and it fit better. I would pick the park that works best with your overall plan.
If taking Disney transport, or staying at an Epcot resort, the discussion ends here.
Otherwise, if driving to the parks: then you have to consider parking. For Bon Voyage, your choices are to park at HS, park at Boardwalk and pay for valet parking, or (if your ADR is 9:30 or later) park at Epcot and walk through Epcot to get there via International Gateway.
If you are staying off-site, paying for valet parking is not much more than paying for parking at the theme park. If staying on-site, then paying for valet parking vs. free theme park parking is quite a difference.