Mares vs Apeks
This is working on UK prices.
In the UK the mares regs are upto twice the price of the Apeks 's. In terms of performance they are roughly equal. If anything the Apeks TX 100 is better than the top of the range Mares, despite the Mares costing the equivalent of $1200 compared with $450 or so for the Apeks.
I don't quite know where the Abyss fits into their product line, but I think it is aproximately equivalent to the Apeks TX40, although poorer in performance. There is an interesting review advertised for the next issue if 'Diver' magasine where they are comparing the $1200 Mares top of the range, with the top of the range Poseidon (techies favourite) costing less than $350. I think (at UK prices) Mares are significantly overpriced.
From the general opinion of people here, you can't go wrong with an Apeks reg. certainly with Apeks you get excellent performance, for a price below some of their main competitors.
I have used Sherwood Blizards in the past, but I (purely personal reasons) don't like them. I have no objective test results, just a feeling from having tried one. Also, their 'dry bleed' system, although it works quite well, has always apeared as a 'bodge' rather than a genuine solution to sealing.
Appologies if my FRF to £ to $ conversions have screwed up, or prices etc.. are different in the US,
Jon T