Between the 5150 and the JVM, I'd have to say the JVH too because it's far more versatile. Of course if you never, ever turn the gain off or down then maybe the 5150. I find the 5150 tones to be generally pretty harsh sounding though, and I do agree they're a tweaking nightmare.
I'm really not familiar with Jet City amplification so I can't say much here but I will say if the biggest drawback is that it requires a boost to get higher gain, don't let that be a detractor. There's plenty of boost options out there today, so pick up a few and find which one you want (or swap them out for different sounds).
The JCA50H has 6L6's which means it should have more low end and remain more defined at higher volumes but it won't get the same sounds when cranked as EL84s either. For that reason, 6L6's are generally preferred by many metal players looking for very heavy tones, but they don't get the same tube saturation either. If you liked the JCA22H as it sounds like you did, my advice would be to just get that amp. I've thrown boosts into amps far cleaner than that to get the kind of distortion I was after, so I'm fairly confident you'll be fine.
I'm not a fan of modern day Marshalls at all -- too fizzy sounding. Generally, Marshalls since the JCM 800 vertical inputs don't do it for me anymore with few exceptions, so if you didn't like the JVM I understand. Also, most of them aren't really voiced for extreme metal out of the box. The newer ones try to do everything but don't do any one thing really good at all. Do remember that most of the Marshalls in metal are JCM800s, and 95% of those have been heavily modified as well to get the sounds that they do.
