Exhentais great for finding untranslated doujins and h-manga. If it's translated, you can generally find it on the scanalators site, or on a seperate, reading site, assuming it's been uploaded, but most untranslated stuff ends up on exhenati first.
And as for the difference between e-hentai and exhentai, you can't access things like loli, shota, guro, ryona, scat, ie:, the more hardcore, and/or morally/legally debatable depending on your place of residence tags on e-hentai, which is open to search indexing, and everyone has full access to. You need an account with access to exhentai to access doujins with any of those tags.
One more thing that sets e-hentai and exhentai aside from other sites, is their download system. You either have to grind in their Hentaiverse minigame thing to earn credits to pay for the downloads, contribute to their collection by uploading and earn credits that way, generate torrents and earn them that way, or help to contribute bandwidth through a separate system to earn free downloads.