Mezz joints
The main difference in the 2 joints is actually the feel of the cue.
The 'United' joint is basically the same as most manufacturer's proprietary joint size of a 5/16-14 thread. It is the most popular, purely because it is available on the most reasonably priced models. >More have been sold at the lower prices.
The 'Wavy' joint is a larger pin that is directly embedded into the timber of the shaft. This gives the cue closer attributes to a one-piece cue. It has a very firm/tight feel when screwing into the shaft.
Those that have tried both styles would normally favour the 'Wavy' joint. It has a much more positive or direct feel, as does any manufacturer's joints that embed directly into the shaft's timber.
This does not mean that the 'United' joint is lacking in any way. It is simply a Mezz option for attaching the shaft to the butt. An advanced version of the 5/16-14 thread is one way to look at it. Mezz do make some models with a basic 5/16-14 thread as well.
Personally the 'United' is a better joint than the 'Uni-Loc' which was developed to be a
quick release joint. My opinion is based solely on the fact that more threads are holding your cue together, giving a better transition of power between butt and shaft.
It is possible to match the 'United' pin to some other maker's shafts that have the same size thread. This is NOT possible with the 'Wavy' pin.
Mika and Alex are both Mezz Pros. Both their signature models are 'United' joints.
Most of the top Japanese Pros use AXI-series and ACE-series cues which use the 'Wavy' joint.
Some have Exceed models which are a Mezz custom option which is available with
either style.
The Mezz website has a lot of useful info if you are interested -
http://www.miki-mezz.com/world/