Different strokes for different folk, there's enough soundclips on youtube.

I've owned both and while I preferred the El Grande, its easy to see the attraction of the Big Muff. The El Grande sounds a lot closer to a heavily overdriven tube amp while the Big Muff is closer to synth. The Bass Big Muff is more versatile as it has a blend function which the El Grande is sorely missing, all there's a £40 price difference which is verynotnice.
They're both intended for different things. The El Grande is very much a one trick pony kind of pedal, the Big Muff is more of a well-rounded general fuzz pedal. If you don't know exactly what you're looking for, go for the Big Muff, if you do know what you're looking for, and it happens to be the same as the El Grande, go for the El Grande. Or, try them both out.