isolation is ok...not the best, but reasonable.
i think my DT-770 Pro-80 isolate better...that's another option you might consider. they don't fold up like the M100, but they are built like a tank, so could be used as a portable headphone (i used to use them as my portable before i got the M100). i think i paid about $150-$170, but can be had for $130ish or so i believe.
otherwise, yeah, UE6000 is a great value option. not sure how the isolation compares (i haven't really compared them all for isolation)...but with the ANC, should do the job. and they do fold up for portability (not as small as the M100 though).
DT-770 = built like a tank, probably the most sturdy. probably has the best passive isolation and the most comfortable.
M-100 = a sleeker convertible tank, that packs away the smallest. probably the least isolation, but sounds the best to me.
UE6000 = probably the most fragile of the 3, but does fold for portability. probably has the best isolation in active mode. probably the best value for money.
so, depends what you're looking for in a portable/travel headphone.
for me personally, the sound and smallest packed away size trumps isolation/comfort/everthing else, so i have been using the M-100 almost exclusively of the three as a portable. although i sometimes use the UE9000 (which is pretty much the same as UE6000 sonically) as a bluetooth portable.