I've been negotiating with that problem as well, except I've also had to handle having large ears and seeing my options and choices diminish dramatically as a result.
In the end I've gone with the AKG K550—they'll arrive later this week. They have large earcups, effective isolation, wide sound stage, and detailed precision in the width and depth of the field. I wanted an all-round set of headphones for everything, but eventually I had to sacrifice on portability and go with the K550's. The Philips L2 were also a viable option as apparently they possessed a wide and deep sound stage yet isolated noise better than the X2 or other open-ear equivalents.
If you have small or 'regular' sized ears, the world is your oyster. Basically anything will be effective, especially ones with a renowned sound stage and neutrality to the signature. The NAD VISO HP50's and PSB M4U1's are an example of this. You mentioned the Momentums... they're supposedly very good headphones for a variety of styles, but I don't know how functional they would be as a gaming headset. I can't imagine the small earcups would be comfortable for lengthy gaming sessions.
You also should consider the Audio Technica ATH-A700X and A900X models.
Im pretty sure I fall into the small/regular size of ears and I know i have a smaller head than a lot of folks (wear size 7).
I dont live near any place where I can go and try these on so I have to try to figure out what i might like just from reviews and such. For me the most important feature would be to be able to hear distinction between the sounds and have crisp sounds. I think that something that allows me to listen to classical music I think would work well for me as for gaming I would like to hear everything as clear as possible over say, having a bass heavy headphone.
the issue i run into is that for every person who knows what they are talking about who likes a given headset, there is another person who is just as qualified who doesnt like it. This is an issue because Im new to the premium headphone market and I dont have a lot of experience trying various models out and being able to interpriute a lot of the reviews other than "ok he liked it and it was clear" "ok he didnt like it because this other headphone had clearer highs".
At the moment I am leaning towards the X1s, but im just not sure about the openness.