I have a similar commute on subway/ bus everyday. Between those two I would definitely go for the Cabby GM in Black. No vachetta to get dirty in the city, the ability to wear messenger style, and the zip top are all huge benefits. I have considered a Neverfull but honestly, the open top of my Petit Noe bothers me enough on public transportation, I don't think I could be comfortable with the Neverfull!
Adis, I think the Neverfull MM would work well for you...I think the GM would be too big for you.
Have you tried them on yet?