I'm renting a car this weekend to avoid putting ~1000 miles (round trip) on my MP3. For $100, it saves wear on my high performance tires and protects my car from road hazards and accidents. However, 3,000 miles one way is a helluva lot of driving. I drove over 1k miles last summer from Arizona to Texas and it took a lot out of me... if it had been that same trip x3, I would have needed a few days in between to recover. Not to mention the interior of my Golf got all fubar from all the stuff that got shoved into it... headliner got messed up, fold down seats had excessive wear, etc. It wasn't a fun trip at all ... I couldn't even see out my rear view mirror!
If you've got the cash, I vote you travel in style and let somebody else do the work. If you do the road trip route, you will have to not only take off the driving time, but also plan for any contingencies... if you get a flat, lose a timing belt, have an accident, get stuck in a construction zone, etc. etc. And after the 3rd or 4th day eating at restaraunts and fast food, filling up your tank, staying in hotels, etc. ... just some points to consider. Hope you have a good trip, tho, and your car and stuff gets across the country safely and uneventfully.