From personal experience with both, after doing a lot of comparisons between the two, inside and outside. IMO--
Better throw--Eagletac D25C
Better build quality--Quark MiniX 123
Better looking--Eagletac D25C
Better ruggedness--Quark MiniX 123
I feel the Quark Mini 123 with xpg is a closer match with the D25C since they would both have similar LED's and characteristics. The MiniX puts out more over-all light due to xml in a stubby design and the xpg model putting out about equal throw to the D25C. Another note, though minor, is the D25C does kick down about 20L after a few minutes when it comes off turbo. Not really enough to notice, but that puts it almost the same output as the Quark Mini 123 with Xpg.
The D25C is a great looking and great throwing stubbly little light. But I have had issues with mode changing with that model, bad enough that replacement was needed. With that in mind, along with the very sturdy feel of the Quark Mini's, I just feel they are the better light for everyday use. So I say get the go with the Mini 123 with xpg if you want good little light with a little more concentrated beam, or go with the MiniX 123 if you want a little more flood. The Eagletac D25C would still make a fine little light irregardless.