Sell (ish).
That's a broad generalisation which doesn't really address all of the other factors which determine the quality of an artist's music. The inspiration and substance they draw from their feelings/emotions is largely pulled from experiences, and both singers and rappers flourish in environments offering experiences which enrich, complement and massively influence the type of music an artist makes, and the extent to which it's 'true/real/good'.
Besides, both make music and both use their voices, so other divisions probably need to be made within the groupings of 'singers' and 'rappers': how does voice and experience relate to the differences and idiosyncrasies specific to mafioso rap, punk rock, grunge and emo rap etc.?
Because within these frameworks, different genres offer up different conventions and styles which need to be considered when off-setting reasons and causes against one another: a death metal vocalist might be born with the right voice more so than a generic pop singer, and somebody else (from another genre) might be less reliant on a high pitched/deep/whatever voice than the lifestyle they've lived.
I see what you mean, though. Some people just can't sing, and some people sound like they were born with this fcking incredible voice. But I think that sort of gift is rare, and there are always things/people which buck the trends and don't conform to patterns/expectations etc.
Other things you'd need to consider might be the ways in which an artist's voice is recorded or altered during production (auto-tune etc.).
AND! Different people like different types of voices. I know next to nothing about heavy metal, but there's probably a range of voices which are popular (amongst artists and fans) and accepted - i.e. Aesop Rock to Jay Z. Interpretations of the 'right' voice are always gonna vary quite a lot, but I'd say both singers and rappers depend on their voice, to an extent, and still work on their pitch, tone etc. as much as possible to move their sound towards realising their artistic vision. These sort of things play into the audiences which might be specific to singing or rapping, or a style within one of (or both) of these groups. Basically, audience and reception applied to voice and content.
In some senses, you're probably right. The hip hop culture (especially in the past) was really the main thing that cultivated talent. Without that lifestyle and culture, there's no way most people would even try to rap or produce that kind of music. Singers come from all different cultures and backgrounds, but I think kids from other walks of life are aspiring to rap now the idea and culture have caught on in most countries/places.
Most people can rap and sing -some are better singers and some are better rappers, but training yourself to rap (flow/delivery/breathing etc.) is different to developing yourself to hit high falsetto notes/head voice etc. It depends on the individual.
Either way, I have no idea, seeing as I can do neither.
Some rappers need the real life environment necessary for making 'real' music: they take what they've seen and experienced and translate that into music which their demographic can relate to.
Singers draw influence from their environment too, though (inasmuch as they sing about what they've seen/felt etc.). The quality of any artist's music's up for debate, but it's probably accepted that a 'tortured soul' won't sing happy teenage love songs, in the same way that deep/conceptual rappers won't bother with the 'mainstream/bubblegum' topics, and vice versa.
In short, I'd say no. The main reason I'd sell is because it's such a sweeping stereotype, even though I can see where you're coming from - and in some ways, I kind of agree with you (there's loads of other shiet to write about too, but I doubt anybody even cares enough to get this far, so I'll just stop).