I hope I'm answering the right question and correctly. I've been out of circulation for a while and much of this information is of the "use it or loose it" variety.
Regarding the Wiz2, it is what is called a buck/boost converter. What this means is that in cases where the voltage of the power supply exceeds the forward voltage (Vf) of the luxeon, the converter will reduce (buck) the voltage that actually gets to the luxeon. In cases where the voltage of the power supply is less than the Vf of the luxeon, the converter will boost the voltage to that needed to run the luxeon at the desired current level.
On it's face, this sounds as though it is the best way to go, but there are always buts. The Wiz2, like all other converters, has its limitations. Some of these include the fact that it's operating voltage range is from about 2.5v to 6v. If the power supply voltage drops below around 2.5v, the light will fall out of regulation. If the voltage of the power supply exceeds 6v, you'll likely fry something.
The upside of running the Wiz2 converter is that you can use your choice of a single primary 123, a single 3.7v li-ion cell, or 2X123. The 2.5v cut off also serves as sort of a protection against over discharging unprotected li-ion cells.
The downside of running a Wiz2 converter is that it generates more heat than either the NexGen (boost converter) or Downboy (buck converter). Another downside is that when running this converter with a single primary cell (in a single cell light), the 2.5v cut off means that the light will fall out of regulation long before the battery is really dead, so you're kind of wasting half of your battery. I say this because the converter depends on the voltage of the battery under load, and when you put a load on a primary 123, the voltage drops around 1/2v (usually).
Where the Wiz2 really shines is when you intend to use li-ion cells as your primary power source, especially unprotected cells.
You just have to keep in mind that there are always more tradeoffs when you are striving to attain maximum flexibility.
I, personally, find it best to dedicate a setup to a particular use (or power source) and build around that. I usually like to use either a downboy or nexgen, unless maximum flexibility is needed or li-ion cells will be used almost exclusively.
Now, this may be more information than you asked for, but...
I usually use the NexGen converter in single cell lights, whether they are going to be used with primary cells or li-ion cells. I've found that the nexgen performs very well in single cell lights. Just remember that with a boost converter, you CANNOT run two lithium cells. When using a boost converter, the voltage of the power source under load, plus the voltage overhead tolerance of the converter CANNOT exceed the Vf of the luxeon being used. What this means in practical terms is that you can use a NexGen converter in single cell lights running a luxeon with a Vf of J or better with a bias of 667mA or so or greater and feed it with either a single 123 or li-ion cell. If you aren't going to use li-ions, the Vf of the luxeon or the bias (current level) are almost immaterial. The reasons that I prefer the NexGen in this confirguration are that the NexGen produces no noticeable heat and it will suck a battery pratically dry, which is a plus when you're using relatively expensive lithium 123 cells.
The downboy converter is a buck converter and I know that it is good with power supplies up to 12v (probably beyond, but I've never had the need). This is a great converter. It produces very little heat and is efficient. You'll get super long runtimes (usually) in lights running at a reasonable current level. The downside is that lights using this converter are usually larger multi cell lights.
All of the above assumes the use of a Lux I or III.
You will likely also see many references to Vf. As a practical matter the Vf of the luxeon will have little to no impact on the performance of a regulated light. Where it does make a difference is in runtime. If I had to guesstimate, I'd say that in a two cell (6v) light configuration, you loose roughly ( and I do mean roughly) 10 minutes of runtime everytime you go up a step in Vf. If you used an H Vf luxeon as a standard and you got 1 hour of runtime, then going with a J Vf luxeon will only get you 50 minutes, going with a K Vf will get you 40 minutes, etc.
