It is significantly different. For one, the Marshall cab. is much better quality, made with much lighter hence higher grade wood( baltic Birch) for classic rock and metal , made with a higher grade Ply, otherwise the 6505 wouldn't weigh a gosh darn ton IMO!
It's not hard to find a lot of reviews on the infrawebs.
The 6505/5150 cabinet is a darn good cab imo but most suited for modern hard core grind or the later Van Halen total brownout sound Ed spec'd, and in the general consensus on web reviews are favorable to the cabinet and the speakers within for such.The Sheffields ( speakers), like in my old 5150 cabinet are better suited for more modern hard core Metal, and voiced for the 6505/5150 heads which have a ton of Low end looseness when the resonance switch is engaged.Those Sheffields ( and I think there is also a different Peavy speaker utilized for those cabinets, get a undeserved bad rap because they don't sound exactly like Celestions,or people are trying to get a classic greenback sound from them, again, IMO.
The Marshall cabs(1960 models) are best for classic or classic heavy metal especially with Greenbacks, but with V30's r G12t-75's can really get into modern metal territory IMO, but you are still better off in that vein with a Bogner, Mesa, or Orange .