Which type is better?

[chipmonk]

Which are better? Ball-locks or pin-locks?

[bub]

I find that ball locks are more common and seem to be easier to work with.
BUB

[Push Eject]

Neither are actually better in my opinion.

Ball-locks are easier to work with if you want to interchange your fittings with 5 gallon and 3 gallon cornies.

Push Eject

[DannyW]

Pin locks are better for preventing you from putting the wrong fitting on the wrong post.

Ball locks are far more common and easier to interconnect with equipment from other brewers should you participate in festivals and such.

Other than that they work about the same.

[beer_bear]

According to what I've heard Doc do with his beers sometimes the ball lock sounds like the way to go. There are a few shows where Doc describes using the "beer out" tube to help carbonate the beer faster. You could also blow CO2 through the beer out tube to unclog stuck hop debris.
I'm going to get the ball lock style when I get to the kegging.