Think of them in the same terms as lip gloss versus lipstick -- one's sheer, the other's more serious. A tinted moisturizer is a face lotion with a hint of color that evens out tone but still lets skin show through -- as lip gloss does. Foundation, though, like lipstick, has more pigment and allows for an opaque "done" effect. So, do you need both? No. The one you need depends on how much coverage you want. If your complexion is clear but you need to even out redness, then tinted moisturizer is your best. If you're looking to camouflage acne scars or pigmentation problems, then opt for a full-coverage foundation.