plastic vs. metal lock closures-plastic is cheaper, lighter, and doesn't bend when stuff gets placed on it. Metal doesn't break when stuff gets dropped on it. Most people choose metal because of this, myself included. I have seen both broken plastic buckles and bent metal buckles. Neither are obviously functional at this point.
"normal" vs. fastex *what those click buckles are actually called*
Fastex buckles are prone to breaking when mishandled, especially when it's really cold out and they tend to get a bit brittle. Conversely they tend to release early when it's really hot out and the plastic is a bit more flexible.
1" webbing fastex are rated for far more pressure than the scuba gear is going to put on it in normal use. About 70lbs for the cheap ones working with a 200lb break, and about 4x that for the "nice" ones. Not going to ever see that in normal use. 2" webbing that we normally use is going to make those buckles a non-issue. They just don't break unless abused. I have heard of several breaking, but all were under abnormal use circumstances i.e. heavy shock load, and/or were abused.
Fastex buckles have 2 major downsides over the "normal" latch type buckle.
First and foremost is that they can slip fairly easily on the adjustment sides. It doesn't take much to get them to slide if there is any tension outside of the plane of the buckle or if the tension goes into a shock type scenario. I.e. quickly pick up a set of gear and the shoulder straps can shift. This isn't really an issue in the real world, but it is something to be aware of.
This leads to their main advantage. They can be cinched while buckled. The latch type has to be released before you can adjust it. Pros and cons.
Second concern with Fastex is that they are typically considered a permanent fixture. If they are a one side adjustment *most scuba gear uses one side adjustment*, then the fixed side has to be secured. This is done at the factory by sewing the webbing over, and can be done by the end user with a tri-glide. This is relatively annoying.
The adjustable side needs to have a tail of some sort and preferably have that tail folded over itself and sewn so it doesn't slide all the way thru the buckle. This also is ideally done with somewhat flexible webbing so it adjusts easily which means you now have something flapping around. You can obviously tuck it away somehow, but that's extra steps.
So, the latch type buckles that are "normal" allow the user to easily thread the fixed side into the buckle and make adjustments relatively easily. You obviously don't want to move that buckle on a regular basis if you can avoid it as anyone who has done it before can attest, but it's not terribly difficult. They can go easily onto any 2" webbing, whether flexible or rigid, and only require one side of the webbing to have something on it.
The "failure mechanisms" of the Fastex buckles are really not something that I would use as an argument against using fastex buckles, but since it does happen on occasion, it is more of the sprinkles on the icing on the cake.
