The 200c and the 500c about the same volume? The 200c @ 140 watts 115 cab vs the 500c @ 350 watts 210 cab? w/o ext cabs. IDK.
Yes, but same sized cab means you're limited by cab size, too.
And what I say can be backed up by mathematics. You can see on various specs sheets that 15" speakers often are 4-6 dB more efficient than similar 10" speakers (BP1525 is about 4.8 more than a BP102), which isn't entirely canceled out by having two 10" speakers (which brings you back up about 3 dB in loudness, except they're sharing cab space so they don't quite get all that). You'd expect that. In any case, 1x15" has more surface area and much more volume than 2x10", so it's actually pushing more air, not less, at the same wattage. The same amp pushing into a 1x15" will generally move more air and more efficient than the same amp pushing into 2x10", even though the 2x10" could sound "louder" because it might be more efficient in the upper midrange where your ears are more sensitive. Sure, 350 watts sounds way more than 140 watts, but 2.5x difference in power translates to a 4 dB difference (log 2.5 ~ 0.4).
So when you math it all out, in combo form, the 200c and 500c are only different by around 2 dB when you just consider speakers and amp wattage, and then there's the cabinet, which being equal probably completely cancels out that difference, because the cab can only give out so much volume. For all intents and purposes, I think I can safely say that the 200 and 500 combos are exact same perceived loudness. Besides, it's not just math. I've had both and I can absolutely tell you that this feels true. But once you start adding external cabs, then they really start to differentiate because the 500 can truly feel twice as loud with a second cab, whereas the 200 will only feel somewhat louder.
But I'm not saying this to say that the 200 is just as good as the 500. I say buy whichever has the speaker arrangement you want, because in the end, that's all that matters. The 200 isn't a smaller amp than the 500. It doesn't have less volume, it just has a different tone due to the speakers. There's no such thing as the 500 being too much amp if you're also willing to consider the 200, because they're the same volume. They sound and feel different, and if you never intend on using an external speaker, you'll never really perceive a volume difference.