Mosfet amplifiers are efficient, but the term Mosfet, is abused in Audio.If an amplifier doesnt have a good power supply, or large capacitors inside of it, mosfet output devices dont mean crap.
Plus alot of companies use cheap Mosfet output devices.
I like Mosfets, if they are done right but I prefer the TO3 ,because they are very reliable, and have a really smooth sound.
Plus they usually have more dynamic headroom,so your amplifier wont go into "clipping" as fast.