The chassis is a cheaper, thinner stamping, fewer parts, so fewer options, two channels, not three, tiny 6 inch reverb tank mounted inside the chassis (it goes "BOI-I-I-ING"), two vs. four power tubes (fewer parts again), smaller OT, fewer controls (only one gain control) so not as adjustable (the more you dirty up the clean sound, the more outrageously dirty the lead channel gets), 50 watts vs. 60 (or 75 Simul-Class).
They do sound great, tho'. Lighter to carry and easier to dial in, if you can live with the fewer options. Maybe even a better metal amp. To me, the gain at max is tighter than a Mark, if less complex. Suum quique!