I voted for the TX-100; I dive both a Zeagle 50D and a TX-100, and there's a noticeable improvement on the TX-100 especially at depths exceeding 100 ft. As you probably know, the 50D and TX-50 are very alike, as are the Flathead 6 and the TX-100 - Zeagle used to OEM Apeks until a couple years ago. Performance-wise, very similar when comparing apples to apples. I like the hard purge on the Apeks 2nd stage though, rather than the soft rubbery Zeagle 2nd stage on the 50D. The hose routing with the TX-100 is definitely very sweet and looks great, but the non-standard port for the high-flow primary reg can cause some problems if you don't have the adapter for it. I haven't compared performance once you put a 7' hose on another port so it might not be as good (until I get an adapter or the port plug for the high-flow port, the TX-100 is on my left post). I'm gonna get another one when I can afford it.
Just fyi from my own talks with cave and tech divers, they all recommend one of three brands - Apeks, Scuba Pro, and Poseidon. Zeagle might be there in performance tests and will be up there in tech use in a few years too most likely, but as far as battle-tested now, it's one of those three. Poseidon is awesome as far as performance, but historically difficult to maintain from what I've heard. Scuba Pro is terrific as well, but not great for cold water diving. Apeks performs pretty good, probably least well of the three honestly, but has a great track record for reliability and easy service, and I'm told they perform pretty well in cold. I'm not crazy enough to dive where it's cold so I don't know.
