I tend to think milsim involves somewhat more 'role-playing' - and that's what I think makes it more interesting. Normally, skirmishes like the games at Flag Raiders are more relaxing, sometimes more social events.
If WP can be seen as the milsim end of the spectrum, then SD can be said to be on the opposite end. If anyone's attended games with WP members and SD members, you'd usually see a difference. WP is usually more 'aggressive' - as Jay puts it - and work together (quite well, might I add - they have some great team work); then you can look at SD members, they're usually sitting on their asses having a good laugh and engaging whenever they feel like it. :P
But at the end of the day what can be said? Both teams had fun.