I think the league server would be best to use, to keep things under control more easy.
On the time I think 6-10pm est. (I doubt everyone will want to play pistol fight for 4 hours but I also doubt their will be something going on in the league server right after so ...... and that 4 hrs will give everyone a better chance of making it in to the event even if they just stay for 20 mins vs. a event that starts at 5pm and ends at 6 and you don't make it because of traffic!
Also I've played Karbala several times on the custom map server without it crashing. The problem with the crashes on the custom map server have something to do it the way the map rotates on its own; because me and several other players just kept rocking the vote one night after we played two rounds of each map and the server never crashed (so a quick fix for the custom map server would be to put in an option which will allow players to switch the map right in the middle of the 3rd round..only thing about that idea is its sucks to have a map change right in the middle of the 3rd round but it works for me, its seems to be the only way to play on the custom map server without crashing)