I believe that anywhere outside the UK, the Mares ABYSS is less expensive than the Apeks TX100. I know that I've seen it for less than HALF the price of a TX100 in the US.
And with that in mind, price/performance then completely blows away the Apeks.
Check the reviews around the net. I have seen some bad comments for the TX100, though the units have been improved over their initial introduction. I have yet to see anything bad about the Abyss - except about it's weight. And most of these complaints are actually due to the stiffness of the hose. The first stage is the beefy heavy bit.
Of all the regs out there, the hardest choice to make comes between the Abyss and TX100 IMO. And the Abyss is especially nice since it lacks any knobs. Though to be fair, most everyone with the TX100 sets it and forgets it.
Instead of the Mares Mission however, I'd go with something less clunky. I'm sure it will work fine, I just don't like its boot structure.
Bruno