Here's a quick quality comparison of etc2comp, etcpak, and my ETC1 encoder (which is directly derived from rg_etc1 but modified to support perceptual colorspace metrics).
etc2comp was limited to ETC1 mode, with REC709 error metrics, using an "effort" value of 1.0. Looking at the code and comments, this mode favors luma accuracy over chroma, and red over blue.
I encoded 1,572 PNG textures from the corpus I used to build and test crunch. I decoded these images using rg_etc1's unpacker, then computed the PSNR error on the luma and wrote the data to a big .CSV file.
In this graph, everything is sorted by the PSNR achieved using basislib (my ETC1 encoder), because it generally has the highest PSNR of the three encoders.
My encoder in "uber" mode is the best most of the time, but there are a few exceptions which I'm going to investigate. Uber mode is much slower than the other encoders, but that's not the point of this test. All I care about is the max. achievable quality of each encoder.
As expected, etcpak's quality is lowest, and it has a lot of quality dips. On the flip side, it's extremely fast. etcpak's internal design is quite clever and well optimized, and it would be interesting if the author included a higher quality mode.
etc2comp's quality in pure ETC1 mode is a little lower than my lib at its "normal" setting, and noticeably lower in the "uber" setting. (I only show the uber setting's quality above to keep the graph simple.) etc2comp's speed is very good, a bit faster than my encoder in "normal" mode. The code looks clean, and it compiles without a hitch in VS2015 which was nice.
For etc2comp, I would really like to see its ETC1 mode's quality improved. I look forward to trying it in ETC2 mode, to see how much the extra block modes help. Perhaps there is a possibility that a really high quality ETC1 encoder could beat a ETC1/2 encoder (that doesn't try as hard) some of the time. (Maybe - ETC2 has some really nice looking modes in there!)
I've generated several Pareto Frontier scattergraphs, showing quality vs. encoding throughput, and etc2comp's ETC1 mode is in the middle from a quality and performance perspective. (Which is good! It's a very practical, production-ready encoder.) I'll post this once I have more accurate performance data for etcpak.