New Jersey to San Diego

Keep in mind it is all relative -- either of I-70 or I-40 (or the other routes mentioned) all have their charms and attractions. I recommended I-40 over I-70 based on the beautiful scenery (you mentioned you prefer the natural attractions) and the historical sites to be found along the route in the east, as well as the western scenery of New Mexico and Arizona.

That said, you would love I-70 across Colorado and Utah so if the logistics of the trip make that an easier choice for you, then I seriously doubt you'd ever regret it. Remember that quite a few renowned National Parks lie along the I-70 route, with some of the most spectacular scenery to be found anywhere!

You can do I-40 another time -- it will still be there.