The early '01 trucks had the leftover '00 engines. These were all rated at 235/500 even in the manual trucks, whereas the '01.5 trucks got increased power ratings of 250/505 for auto and 275/525 for manuals. I know because my '01 was an early build date and even with the ZF I got the 235hp engine, older style starter, and forged rods. There is a distinctively different sounds between the starter on the later '01 trucks and the '99-'01 235hp engines. The seats in the XLT trucks also changed up slightly with a different fabric and different center console in the later trucks. My '01 was sorta unique in that it had the newer seat fabric/styling but the older '00-style console without cup holder. Nevertheless, it was a very comfortable truck to ride in. With the Edge Evolution programmer and 4" exhaust my '01 would fly. Wish I still had it.
If I was looking for a truck right now, my first choice would be an early '01 followed by a '00 or '99.5 for the most comfort. That said, I would also have no problems with the OBS trucks and their toughness more than makes up for the lack of amenities that you find in the newer trucks.