Yeah, they couldn't be more different.
The Rat actually does target the "Marshall frequencies," but some people can't dial them in to their liking. It's not an "instant gratification pedal" for many users. I frequently know of guitarists who put their Rat pedal in mothballs and only find out how usable it is until years later, sometimes MANY years later.
The Box of Rock is 3 SHO's cascaded together, with some crude design elements that are supposed to sound like a Marshall. Vex "borrowed" the "Marshall elements" from the Brown Sound in a Box II (entire tone circuit, "speaker treble rolloff emulation," interstage "treble peaking circuit") from the Brown Sound in a Box II. The differences are mosFET's substituted for the jFET's, and you get another SHO that you can switch on, if the Marshall dirt tone is not enough.
The reason why the BoR sound more like an amp emulation is because Ed Guidry (the designer of the BSIABII) went with the treble peaking circuit and the speaker rolloff emulation that is taken directly from a Marshall design. I think Ed was more targeting plexi sounds, though. He was mainly shooting for EVH sounds, hence the name.
Here's clips of the Brown Sound in a Box:
...I'm supplying those clips not because I think that the BSIABII is necessarily better than the BoR, but I think it is definitely a bit tighter sounding. IMO, the mosFET substitution in the BoR versus the jFET's in the "original" design make the BoR a bit muddy and flubby; the circuit needs to be altered to work with them, IMO.
It's also why Catalinbread went with jFET's with their DLS, which has a similar circuit (but is nowhere near 1:1 with the BSIAB). I also think that the CB pedals come closer to amp emulation in general.
Possibly the best Marshall-in-a-box that I've personally used is the runoffgroove.com Thor. Here's a couple of sound clips of that:
It's the only one that I've tried that seems to really nail both the "signature Marshall amp qualities" as well as have a gain range from "medium classic rock" up to those EVH tones.
...The only issue with the Thor is that it is mainly a DIY pedal. You can get one built from these guys:
It's only a little cheaper than the BoR though (about $40 vs. a new one). But you have to wait while it's built; they build them one at a time like Paul C. or similar. Also, I have no affil with OLC, so I can't vouch for their service. But the pedal sure sounds super b1tchin' Marshall to me.