Both made by Collins Performance, who bought the Magnex brand when they went under a few years back.
I have one of the pre-Collins Magnex systems on my 3dr and it fits and sounds brilliant. Had a 2 box Mongoose before and it was awful, especially on long motorway journey, too loud and boomy. Magnex is far more refined.
Having said that, my mate bought a Magnex to replace the original on his Escos last year, and the fit was awful, it went back to Collins in the end and even they couldnt get it to fit right.
Dunno about the Mongoose OE system though, suspect if it's made by the same people it'll likely be fairly similar to the Magnex