The HD238 is my current go-to when I'm want to tote a portable, over-ear open headphone (and I'm more of an open-headphone kind of guy). In terms of its sound, I think it's deserving of Sennheiser's HD family name. In my opinion, however, it's not made to be tossed around the tough environs of a stuffed backpack as much as the PX series is. I haven't heard the latest PX100, but I do have the most recent version of the PX200 (PX200 II), which is very nice for a closed ultra-portable over-ear. Both of these choices are on the warmer side, so keep that in mind. All of the PX headphones I've owned have endured some pretty serious abuse--I've found them to be very durable.
As for the recommendation of the MDR-V6: If you like that form factor, I'd recommend the SHURE SRH440 over the MDR-V6, as it has better overall balance and fidelity (to my ears), and further distances itself when equipped with the SRH840 pads. Since you said price isn't an issue, try the SRH840, which, to my ears, is absolutely exceptional for the price. Keep in mind all three of these choices aren't ultra-portable, folding into something more akin to a ball-shape that is more than a handful. (The PX series headphones fold up into something more the size of a set of folded up sunglasses.)
(The Sony MDR-V6 will always have a special place with me, as it was my first good pair of headphones, my first set being purchased when they first were released back in the 80's. I just think there are better choices for the money now.)