I've owned a Mark III for 18 years, and bought a 50+ a few months ago. IMHO the 50 Cal.+ cleans were real close to the III, maybe even a bit "spongier". The lead sounds were not as fat or detailed (no Simul-Class), but still very much in the Mark III ballpark.
But they are noisier, more prone to tube microphonics, and the reverb tank is smaller and inside the chassis.
I would still pay the $200-300 more to get a Mark III, even a 60 watter (with Graphic).
Mark IIIs are seriously under-priced and under-valued, IMO. You can get one for about half what a used Mark IV is going for. Unless you have money to burn, a Mark IV ain't twice as good as a Mark III.
(Flamers may jump in now...)