This has been discussed a bit here before but if I remember the gist of what Taylor said, the SubT is analog and creates the sound by modulating your input in various ways at audio rate, and so depending on what note you're playing the timbre of the "waveform" can sort of morph a bit, whereas with the DubT, it's just generating a waveform digitally so it will be a uniform timbre across the range.
Having tried both, I will say the SubT definitely tracks better but it does have that audio bleed from the uni-synth side.
I just got another DubT yesterday after selling mine a few years back and regretting it. I really do like the extra options on the DubT, and now that I have a better bass for tracking these sorts of pedals I expect I will have more fun with it this time around.