I've had the SRH840 (sold to a coworker) and just got the ATH-M50 (it was cheap enough, I figured why not). I also own the Denon AH-D2000, which I like more than either in terms of comfort and sound, but it doesn't isolate as well, nor is it that portable. (Fits my use case better than either, as they stay on my desk at work.)
I think I might recommend the SRH840 based on your criteria from your list. The comparison between it and the ATH-M50:
* Sound -- tough call as they're pretty different. The SRH-840 tries to be more neutral and the ATH-M50 more colored in the sound. Depends on what you're after
* Portability -- They're very similar in terms of the size and shape, but the ATH-M50 is slightly smaller and thus more compact and more portable
* Comfort -- both make my ears really warm after a while (which I'm not fond of), but the SRH-840 is more comfortable between the two. It's slightly bigger, so there's a big more room for the ears.
* Isolation -- They both isolate decently, but the SRH-840 is a clear and definite winner here. At a reasonable volume, I can put the ear pieces together and there's not a peep. Separate them and you hear the music. At the same volume, I'd still hear it a little form the ATH-M50 doing the same thing.
Definitely not a Grado... I have the SR60 and the SR225i. I love 'em, but they're as open as open can be, so it won't meet your needs on the bus/subway or in a library.
Then again, to be honest, I think you're barking up the wrong tree with your list. If I were you, I'd be looking at earphones like the Shure SE215. They offer way better portability and isolation than any on your list. I also find them to be very comfortable as well. I keep a pair in my pocket wherever I go.