Ok, being the resident compressor afficianado, I will chime in here:
I have had both the Aphex and the Demeter. I currently use the Demeter Compulator on my live setup. For recording I use a UA LA2A, 610 or Avalon vt737.
First, both the Aphex and Demeter are excellent units. Both are quiet, well built and efficient.
Let me point out that finding a compressor, especially am optical compressor, which does not color the sound is impossible. All of them do...and that is the charm. The idea is to find one which colors your sound in a good and usable way. In this matter both the Demeter and Aphex work nicely.
The Aphex is fairly transparent, and quite fast in its response to transients. The DI is a nice add on, as are the switching capabiliites. But if you, like me, have a good stand alone DI, the DI on the Aphex is a moot point (it is good, but not as good as a Countryman, Radial, or my choice a Demeter Tube Direct).
The Demeter behaves more like vintage compressor...in fact it sounds very much like my LA2A. This coloration is quite subtle, but is a nice "enhancement". This factored big into my decision to go with Demeter.
Also...the Compulator is VERY well built. Way above any other pedal type compressor I have seen (and I have seen and used a lot of them)...add to that Demeter's cust. service rocks (the very small staff there are aces....they actually care about the people who buy/use their stuff....I am not too sure about Aphex...)
I think the Demeter Compulator is one of the finest compressors on the market. It outshines, IMHO, many rack mount units...and yes, James Demeter has been designing top notch studio compressors for years (and they cost plenty, btw) and so (and I can vouch after speaking with him a few times) he knows a thing or two about designing a good compressor. And he certainly did with the Compulator.
So my vote (and $.02) goes to Demeter.
Max