I owned the D60 before the D120, after trying out a D120 I sold the 60 and bought it. I felt the 120 just had a lot more "oomph" to it, bigger bottom end, little angrier sounding. Also, as some have stated- bit more modern sounding which is what I was looking for, differences I would expect in some* amps that have different watt models.
That's not to say that the 60 doesn't have an awesome bottom end and sound very aggressive though, just that the 120 had a little something more to it
both are great amps, when I have the cash I will go back to owning both the D120 and the Ultra-lead, but until then, I'm loving the JCM 800 I traded for