It depends a bit on what’s important to you. If you watch the prequels after the original trilogy, then you’ll likely end on a sour note and it won’t lead into TFA (The Force Awakens) as well.
On the other hand, if you watch the prequels first, then several aspects of the original trilogy may be spoiled for you.
My choice would simply be to not watch the prequels at all (or save them for some time in the future). 😄