funnily enough, i own both a Threshold and a TCX. although i've not directly compared an alloy version of each (i have a Threshold A3, and TCX Advanced 1)
that said; the Threshold is a great bike for the money, my only gripe was the wheels were pretty hefty. they seem to have fixed that on the new A1 at least; mine had a 32h wheelset, presumably used on the Norco MTB's. a 28h offering is more CX-ish, and should be a bit lighter.
i swapped the groupset to sram (personal preference) and a lighter set of wheels, and it goes like a scalded cat now. so much so i swapped my carbon road bike for the TCX, as the relaxed geo of the CX bikes suited me better.
the only standout i can see is the TCX has better TRP Spyre brakes, but it's also $200 more... of which would buy you a set of Spyres if you felt they were lacking. my A3 came with Hayes CX Expert (step above CX Comp) and they are fine though. plenty of power, just the modulation isn't anywhere near as nice as the Spyres. they do the job for a training/commuter/backup-CX bike though.