I guess it depends on the mod, but I owned an Angry Hippie & have spend some time on my friend's Marsha and neither (especially the Hippie) share much ground with the Nitro. Only thing I'd say they have in common is the Marshall heritage and it's out the window after that.
The Nitro is more modern sounding & feeling. It's tighter with a low mid voicing, but I wouldn't say it's dark. And if you have V30's it can be very tricky to dial in the high end so that it cuts w/out taking your head off. Unless they've changed it, it's also not near as saturated or gainy. It has more of that clean gain thing going on. I didn't think it was that flexible either. I won't say it's a one trick pony, but....
Assuming you're talking about the usual Kitchen Sink/Marsha Friedman type stuff & not some other mod, the Friedman is much more classic sounding, but can get very nasty too. With a boost you can get as heavy as you want, but still retain that "Marshall-eque" vibe. Much more flexible. Seemed easier to dial in a balance of cut w/out being harsh. Has more of a juicy feel than the firmer Nitro, but it's not saggy on the bottom either. Doesn't have the low end punch of the Nitro (but not many amps I've played have that punch either), but still punchy none the less. Actually the more I type the more I realize how different these amps are. Hope that helped at least some haha
