From AZ to PA and back

Hi! I will be driving from Arizona to Lancaster, PA in late April and returning in mid-late September. I am currently in the process of trying to choose a route . . .
AAA.com's online TripTik recommends taking I-40 to Oklahoma City, then I-44 to St. Louis and catching the I-70 from there. From what I understand, that's the shortest (and therefore probably fastest) route, but I am open to other suggestions. I would definitely like to take a different route coming home than I took going there. One idea is to continue on the I-40 from Oklahoma City to the eastern side of Tennessee and then using the I-81 along the Appalachians. I have not spent much time in that area of the country (other than visiting my ex in-laws in WV!) and I am interested in going there. Has anyone driven the I-81 between Tennessee and Pennsylvania? How recently? Do you recommend it? I was wondering if that might be a good route to take in September, as maybe the leaves will be changing?
I asked for advice on the Trip Advisor forums, and so far no one has told me anything about I-81. One person said they drove from Philly to NV and took the I-95 to the 66 to the 40 and liked that route, but I don't know if that's best for me.
As I am driving to and from a job, I don't have a lot of time to go off the beaten path (I intend to make the trip in five days, arriving the fifth day) but I would like an interesting/scenic route as much as possible. I also would like to avoid bad spring weather (tornados!) if I can.
Another question regarding the I-40--I have heard that it's very rough going through Oklahoma. Has anyone driven it recently? What's the verdict?
Thanks for your suggestions!

Heading east through TN from Oklahoma City could take you into Memphis and Nashville on your way to I-81. I-81 isn't the most scenic route, as it pretty much parallels the Blue Ridge Parkway and Skyline Drive for most of the trip, plus it's pretty monotonous. Any of those routes should be good in September however.