[quote=Dervish]
Depends on the scale and what you're doing. I tend to like open RPs, but I've had some great experiences with invite only games. Normally, I just stick with the tried and true bar of entry to ensure the players are all at a similar skill level and motivation before starting.
[/quote]

 This is the best answer because it does what all the others do in their ramblings in just one paragraph. :lol

In all seriousness though? Whichever makes you more comfortable. I make public role plays but blatantly show personal favouratism towards people I know and can trust in terms of positions of power and limited slots. The "core" group, with anyone else tagging along and giving it a shot if they want to, and if they prove themselves, becoming part of the core for future role plays.