I've tried quite a few "Marshall in a Box" pedals lately, and none of them really sound like a Marshall stack. Of course, there is always the issue of which Marshall you want to sound like. I like a lot of different Marshall sounds, but my favorite has to be the 1967-1969 plexi-panel JMP series.
I found the Box of Rock to be a good sounding pedal. It is said to be based on the JTM45 sound, and I found surprisingly much gain/distortion for a JTM45. I was reminded more of an early-1970s aluminum-panel JMP. The boost is a useful feature. I'm definitely keeping mine.
Some other really good pedals I've found are the Lovepedal Purple Plexi (there are a few different versions of this one; I like mine) and the Menatone King of the Britains.
I haven't heard the Big D pedal you mentioned, so we have reached the end of my ability to be helpful in your quest.
I use the Big D (love it), and have wondered about the difference between these 2 pedals as well...I wanna get a BoR to compare. I'm a huge user of zvex as well...one of the best pedal dudes out there. hopefully somebody else can compare before I pull the trigger. i also am about to possibly do an OCD which supposedly is in that same ball-park. no help from me..sorry...but am equally interested!
I have a Big D and can't sing its praises enough. For some reason I think that maybe my OCD, Klon or TS 808 reissue are going to sound better but I always come back to the Big D as my main overdrive. It just sounds Incredible.