I'm certainly no expert but I lean toward thinking the original recordings were true. If your ideal of an authentic Delta vocal is the deep primal growl of Son House you might find Roberts singing unnatural, but I've always preferred the high plaintive wail of Skip James, even higher pitched than Robert. Think "Hardtime Killing Floor Blues".
Also, some of the guitar on the slowed down versions seemed a little clunky to me, but that could just be because I'm so used to the hearing the originals.