I am looking at these amps as well. From the reviews I've read, it seems that M-Stage is more neutral while Neco Mosfet is more on the warm side. In the end, it might come down to the kind of sound you like and which headphones you are going to pair the amps with.
In terms of features, these amps are actually quite different. The Neco Mosfet is pretty small and relatively light so it's easy to carry around. It has an external power supply that comes with UK and Euro plugs, I read somewhere it's only 220V but it's fairly easy to fit a dual voltage one with various plug adaptors to get a truly transportable amp in case you want to bring it with you when you travel. On the other hand, the Matrix M-Stage power supply is set to 220V and it would be difficult to fit another one without major surgery. On the positive side, the M-Stage has two sets of inputs and one RCA output so it's more flexible in depending on your rig. The build quality and components used in the M-Stage are slightly better than in the Neco Mosfet IMHO.
Finally, from a price standpoint in the UK, the Neco Mosfet with upgraded opamps (highly recommended) will set you back around £150 (inc. shipping) while the Matrix M-Stage will be roughly £230 (inc. customs and VAT). Another important point is that Neco is located in the UK so if you ever need the amp to be repaired, that's a huge bonus.
Hope this helps with your decision. In the end, if I had to chose, I would go for the Matrix with Sennheisers and with the Mosfet V2 with AKGs (but that's just me).