Vassal PBEM or Online Better?

Which is better Vassal online or PBEM. I used to PBEM a lot of Campaign Series (East Front II etc.) and Battleground series ( ACW & Napoleonic0 Talonsoft computer games. I can see Barriers to PBEM a lot of board games because of stuff like the other guy having to retreat his unit during the combat phase so a lot of file swapping. OTOH when you play on line I can see down time being an issue. Even though I like short small games. I have only used Vassal so far to solo. So I’m wondering.