It depends how good your ear is. 5.1 sound uses 6 channels & 6 speakers whereas 7.1 sound uses 8 channels & 8 speakers inside the headset. 7.1 surround sound is better as it has slightly better sound quality, creating greater depth and precision in the sound. Of course, though, 7.1 channel headsets tend to be more expensive.
I'd recommend a good 7.1 surround sound headset for gaming or audio but a 5.1 headset will suffice for general tasks.