They each have their uses, but I generally prefer seeing the flat coloring. Maybe there's a way to use flat coloring for the figures and the painting-style for the backgrounds without making it look out-of-place?
It's way too hard to choose. I like your painting because you manage to give off so much more...atmosphere and feeling, but at the same time it can sometimes look muddled or perhaps even too painterly.
The flat colors can be effective too, and they're clear and crisp and they still manage to convey some atmosphere. But they don't seem to catch the eye, to me at least, as much as your painting, and there could be some feathering that could take place.
I personally think your ideal style is your airbrushi painting, that you used on such pieces as Katrika Vore, Yuri Vore, and several others.
i can see more detailed lines with flat, but you have way more depth in color when you paint. so as far as coloring goes i choose paint. if you could somehow get the best of both worlds in there... money.
Both are good; I think the flat coloring matches your more cartoony works beautifully, while the painting is good for the slightly more realistic stuff. But given my 'druthers, I prefer the cartoony stuff more.