Dingo,
like Wadzii said, UCcnc has much better timing and now has the synchronous M205/M206 THC ON/OFF commands which Mach doesn't have unless you get a packaged system like Candcnc.
UCcnc is young but the developers seem very commited to improving it and are always working very hard. As an example when the UC400ETH first came out I immediately grabbed one and started doing lots of bench tests, injecting signals, measuring outputs, and testing timing down to microseconds with a logic analyser. To my horror one of the things I found the did not work was the M205/M206 THC ON/OFF commands. I immediately contacted Cncdrive and they confirmed my findings. They said this was going to require a complete rewrite of that part of the code and it was going to take some time to do. Despite the main software developer having a lot on his plate at the time with non business related issues the code was re-written and now works great. There were other bugs I discovered and reported and they ALL got fixed. Was I a bit disappointed that I bought a system with features that didn't work, yes a bit, but the flip side of the coin was seeing how the company sorted them all out ASAP. That's the type of manufacturer I want to go with.
Recently I mentioned to Cncdrive that if I was to buy a Neuron THC I would like TWO synchronous laser outputs, one to use for synchronous control of the torch and one as a synchronous THC enable output to the Neuron. They said it will be easy to add another synchronous laser output. Try getting any modifications to Mach3.
So this is the question you've got to ask yourself. Do you want to start your journey with a new modern system being continuously developed and improved as we speak, or do you want to go with an old buggy system with no further development. What you get is what you get. Regarding Mach4 just have a read through the Mach3/Mach4 forums and you'll find lots of complaints about how Mach4 is only half developed yet they've put it out there for sale.
UCcnc also has a Mach3 plugin for the UC400ETH so you can also use Mach3 if you want. I've just got a response that the new UC300ETH will also have a Mach3 plugin written for it too, once the final development is finished:
http://forum.cncdrive.com/viewtopic.php ... 7&p=83#p83
As for plugins and 3rd party manufacturers, I can't really say yet. UCcnc is still young and hasn't "exploded" yet but I've been calling it the Mach3 replacement for a while now, and I think it just a matter of time before it's going to be the one people are going for. As it grows then no doubt the 3rd party add-ons will start coming along.
The support forum at Cnczone is slowly gaining momentum but Cncdrive have only just got their forum online (just joined yesterday) so no doubt a few existing posters will go there from now on. Have a read of those too.
http://www.cnczone.com/forums/uccnc-control-software/
http://forum.cncdrive.com/
There's also a guy called Terry Parker (Vmax549 on the forums) who has taken UCcnc as his Mach3 replacement. Terry is a plasma cutting veteran and is extremely knowledgeable on many thing CNC related. I'm certain he does not sleep because that would take away from him playing with CNC stuff. If he has moved to UCcnc for plasma and other tools then there must be a very good reason. Previous to this he was EXTREMELY active on the Mach3 forum but now seems fully focused on UCcnc.
Good luck, I know it's a hard choice.
Keith.