The trouble with these comparisons is CMSS-3D Headphone uses a generic HRTF. If your individual HRTF doesn't match it, it's never going to work that well. If your HRTF is close it may well blow you away.
I too prefer Dolby Headphone for gaming, having used a Xonar D2. According to Dolby, DH is not HRTF based so makes for a more consistent user experience.
Having said that - I went back to X-Fi from Xonar. Why? Well I missed some X-Fi exclusive features that weren't available on Xonar (I'd owned an Xtreme Music before - I now have a Prelude). I've heard MacroFX and Elevation filter make a significant difference in games. There were a few legacy games with well implemented EAX3-5 (it's just a set of tools after all) that I felt were worth having. There were compatibility prolbems with some of my games. It took too long to patch Bioshock and Assassin's Creed. Mass Effect was an audio mess whenever I had a Xonar. Asus seemed so slow to fix any problems I became disillusioned and decided it was unlikely some of the problems would ever be fixed.
So what was my solution? For single player gaming I went for a Victor SU-DH1. As Audioholic indicated, there is delay with this solution - enough to be a problem in multiplayer. You can't just send 5.1 Dolby Digital Live from the X-Fi for gaming though - at least not optimally. Elevation filter and MacroFX stop working at 5.1 or above so I send 4 channel sound over DDL to the SU-DH1. Depending on which headphones I'm using I either use the SU-DH1 as a pre-amp into a headphone amp or use it as the amp.
Headphone synergy with Dolby Headphone is important. Detail and accuracy are important. Angled drivers on headphones are, however, a bit of a problem. Dolby headphone is designed to work with no headstage enhancements. Dolby Headphone with straight driver headphones is a bit like having 5 speakers perfectly positioned around you (the '.1' is mapped to some of the satellites). Angled drivers, like on the HD555, ATH AD700 are a bit like having the speakers on each side rotated towards the front centre. It's not a deal-breaker but it just detracts slightly from the experience. Generally for gaming it's not that bad - I noticed the effect most when trying to simulate stereo speakers with 2 channel DH for music listening. I do think that may be one reason why Audioholic prefers the DT880s over the HD800s here.
For multiplayer I'm either putting up with the SU-DH1's delay (casual multi) or switching to CMSS-3D Headphone (competitive multi). To me Dolby Headphone, combined with the additional X-Fi positional features is the most immersive game experience I've had. My ideal would be an X-Fi card with Dolby Headphone, or at least a receiver with imperceptible delay.
Having said that - if I was to just have the soundcard plus headphones with no additional equipment (such as gamers on a budget might be restricted to) I'd choose Xonar.
On Dolby Headphone mode - I use DH1 most of the time. DH2 and DH3 simulate room reflections. The trouble is many games (certainly EAX ones) have room reflection or reverb effects. The two together is nothing short of a mess.
Edit: when I had my Xonar D2 there was delay using Dolby Headphone. I found out this when I was trying to monitor my electro-acoustic guitar. I like to drop in a bit of percussive body slapping and it was at this point that it became painully obvious that Xonar D2 DH was not instantaneous. Turn dolby headphone off and there was still a little delay but not anything like as much. It's not out of the question that Asus may have fixed this with a driver update I suppose.
A few favourite headphones for Dolby Headphone usage in no particular order: Goldring NS1000 - Goldring DR150 - Goldring DR50 - Senn HD580, Senn HD600, Senn IE6 (I was surprised at an IEM working well but it does the job!). I've tried all the headphones in my sig with Dolby Headphone (been using it a few years). If they aren't mentioned above take that as a 'not recommended for DH use'.
Anyway - surely the nearest thing available to a 'Holy Grail' is a properly calibrated SVS Realiser? Any delay on that? I've established from testing elsewhere that the delay with DDL encoding isn't too much of a problem. The delay I've experienced has been mostly at the receiver end.
Further Edit: Chinesekiwi mentioned close quarters. That's why macroFX is important. It's for simulation of sound close to the listener. As indicated above - you can have it with X-Fi and Dolby Headphone as long as you stick to 4 channel mode for DDL or DTS Connect.
Anyway - Audioholic - thanks for the write up. It's been very handy to hear your impressions with some higher end headphones and Dolby Headphone.