You need some history insight.
Before windows XP NO PE was possible (for a lack of a switch in the kernel).
Initially Microsoft provided to it's partners a Preinstall Environment (versions like WinPE 1.2, 1.5 and 1.6 were given to OEM's and selected customers.
Any PE built with Microsoft tools/approaches is called "WinPE".
PE by itself means normally a Preinstall Environment, including the WinPE's.
You had no choices available (and unless you were one of the elected ones you could not have the only PE existing, the WinPE).
Bart Lagerwej created a program (actually a sort of processor, not entirely unlike batch processing) called Bart's PE Builder, capable of creating a PE very similar to the original MS one.
The project (just like the original WinPE) was intended for "emergency" or "install" or "deployment" ONLY.
As such the number of applications running in it were the few needed ones and the interface (at least in WinPE) was essentially a command prompt.
Little by little the project was expanded and in practice you can have running in a PE *any* app that you can run on a fully installed systems.
The approach by Bart was a highly modular one, you added the programs you wanted to run (through plug-ins) and each BartPE build is usually "highly personalized" by the builder.
The "base" inteface is made through a program nu2menu that allows to run the BartPE also from a booted XP (thus taking advantage of the programs in it on an already booted machine).
The UBCD4WIN project (which uses Bart's PEbuilder as engine) is essentially a set of programs (and plugins for them) pre-assembled and "harmonized" in a "generic" PE disk (still mainly aimed at "emergency").
Sherpya's XPE introduced a "paradigm" shift introducing an interface that is virtually indistinguishable from an installed XP (still using Bart's PEbuilder as engine).
Several years later a young portuguese guy, called Nuno Brito started fiddling with a program, originally called Batcher, that later evolved into Openbuilder and that is now Winbuilder.
Winbuilder (as the original Batcher names suggests) is a scripting engine, very similar in concept to the batch capabilities of CMD.EXE but GUI, which provides "total control" over the processes.
The difference with Bart's PEbuilder is that the latter has a part "embedded" in the executable (and thus not changeable, whilst in Winbuilder everything is in the .scripts.
If you prefer the main differences are that PEbuilder is a scripting engine aimed to (and limited to) build PE's from XP or Server 2003 sources (PE 1.x), while Winbuilder is a more generic scripting processor that can be used (though it is normally not) for *anything*.
Due to it's "generic" capabilities, Projects (that are sets of .scripts pre-assembled and hopefully optimized) have been developed to build with it, besides PE 1.x (XP or Server 2003 based), PE 2.x (Vista or server 2008 based) PE 3.x (Windows 7 or Server 2008 based) and recently PE 4.0 (Windows 8 or Server 2012 based).
Additionally "specialized" projects (outside the original scope of "recovery" only) have been developed for Winbuilder including features like multimedia, Internet browsing, Wireless network support, etc., etc.
BartPE can be directly compared to Winbuilder (though as said thus latter has far more capabilities)
LiveXP (particularly in one of Amalux's Frozen builds is directly comparable to UBCD4WIN
If you are starting playing with PE's the two recommended things (for XP/2003 sources) are UBCD4WIN that "builds for everyone" as it has very little "choices", or one of Amalux's Frozen builds (that require as well the least amount of "fiddling" by the final users):
This latter is possibly "better" in the sense that you will get familiar with the Winbuilder tool that as said has evolved and allows - unlike Bart's PEbuilder - to build (using other projects) later versions of PE's.
Bart's PEbuilder and Winbuilder are not the only builders around and in some cases a "portable" XP or Windows 7 is preferable over a PE, no matter how "expanded outside the mere use as emergency tool" it is.
So, maybe, if you could list your needs/desires, it would be possible to point you to a specific project fulfilling them.