You will get a great deal of debate on this one... I prefer whole food sources... Acerola, when we are out of citrus season here.
When in citrus season (I am starting to get limes now), I use quite a bit of citrus through juicing, spiking my water, making honey lemonade, eating oranges and grapefruits.
You get all the rutin, pectin, quercetin and other cofactors, such as the alkaline minerals that are so vital to C function and efficacy that you would NEVER get in an ascorbic acid.
One study I read, which can be found on the "Innate Response" web site, showed where 5mg of a whole food C (eating the whole food - in this case an apple) had the same effect/efficacy as 1,500mg of ascorbic acid.
While I did have one individual point out a problem with the study (any study has faults), the idea is sound and accurate IMO.
A whole food C is more efficacious than an isolate.