tax efficient umbrella companies v standard umbrella companies

Hi, just need some advice as to what I hear are called 'tax efficient' umbrella companies (also known as alternative umbrella companies) as opposed to standard umbrella companies. As a locum worker I am currently using a standard umbrella company but having secured a new assignment my agency has given me a name of a 'tax-efficient' umbrella company.

I have spoke to the umbrella company directly and they state they are based in the UK therefore not off-shore. They are able to provide me with a net weekly pay significantly higher than a standard umbrella company. They say they do this via applying slightly differing rules than a standard umbrella company which they say are totally compliant with HMRC guidelines/UK laws. Seems almost too good to be true though I am tempted to go with them given the increase in my net pay compared to a standard umbrella. Anyone know whether these tax efficient/alternative companies are legitimate - I don't want to register with them only to find out the HMRC regards going with them as tax avoidance and asking for me to pay back monies I saved via going through them.

Thanks in advance