The truth is there is very little discernible difference between one pre-spray and another, at least not on your average lounge carpet, as there are so many other factors at play beyond the pre-spray used, colour, construction, fibre type, age, type of system used, temperature of the cleaning solution etc etc so it is difficult to attribute the result to just the pre-spray.
Pre-sprays play more of a part when the carpet is particularly dirty, like greasy restaurants, dirty pubs / offices etc, and then it is more important to choose, or have the choice of a range of pre-sprays and not just the one that got Mrs Jone's carpet clean, the one that fooled you into thinking its was the dogs danglies in all circumstances, when in fact it is not.
I don't like M-Power because it falls short of what other pre-sprays do by quite a margin.
Simon