welcome aboard!
I had both for a short time, but the Treadplate I sent out for a PeteMod and the Modern I had to sadly sell off during a money crunch.
from the stock experience:
They have similar NuMetal tones, but I think the Modern may have been better at lead-more mids, sustain, and wholeness to the notes, while the TPlate was a rhythm crunch machine. They may have had different bass caps stock, which may also be the main reason for this (tight/loose), and that's a mod many can do on their own.
Similar ballbark, probably why the Modern was discontinued. Dare I say the modern was a bit of a cross between the SL+ (not the 800 version) and Treadplate? I don't know, the modern's cool because it is getting to be a hard item to find, but the Treadplate is a better piece- especially when Grail modded or better yet fullPete moddded with the Orange switch.