There shouldn't be much aside from cooking and screen size. This being said, in the U.S. at least the jump to the E14/P15 gen1 on the aftermarket isn't that much in terms of price and the performance has a significant improvement.
The full T14 gen 1 goes for about the same price as a T480. Unless you really crave the dual batteries or dual ram slots the T14 is a better machine.
T480 (not sure about T580) is a strange beast regarding storage implemantation. I've seen SATA models (worse, slower) and NVMe models (better, rarer?, faster). I own the SATA one and I believe that good part of the performance difference I see between it and a T14 Gen 2 (running same setup and same tasks ) is on account of the faster storage. Among T14 devices NVMe is I believe standard.
Don't all t480 models have both sata and m.2 (made for the smaller 2242 form factor) slots? Or are there full size nvme variants instead of sata?
I don't think it's the case for the T480/580, but for the T14 and T16 machines it's literally the exact same mobo just stuffed into the larger case.
You can get a bigger battery on the T16 at least vs the T14
IBM and Lenovo ThinkPad laptop enthusiasts!