In my experience with my MB150E, it's pretty tough carrying that and my bass up and down stairs, through the emergency doors, and into subway cars (let alone finding space inside the subway for all that). I have a case for the amp and a nice, light cart (b&h photo for that) for when I'm rolling it, and I'm a pretty big guy, but still, it's not easy at all. I know it weighs 26 lbs, so that and its shape can add up when you have your bass in your other hand. Having said all that, I'm able to get the right kind of sound out of it (it stays true to my acoustic sound), so that's good. But taking it on the subway is not easy at all for me.
I've been thinking of checking out one of the AI's that are out there. I feel like they're a bit smaller than the GK, and I think they're right around 20 lbs, which would make a big enough difference. As long as I can get my sound out of it, and if it does make it easier to take on the subway, I would get it.
One thing to check out is the dimensions of these amps you're looking into. I wonder if you could do a test run with someone's MB150 and your bass, just to see what it feels like. Good luck though.