The cup seal valve seals better - but is harder to install - using 14mm by 2mm
I've always opted for the harder install personally. Once it's in there you don't ever really have to take it out if the seal is good. That's also why I've liked using dow55 on the valve orings. Lock that sucker down and play paintball.
Mint Infamy Creed F/S ( needs battery/ button pad).