I'm seeing that the 901's are really popular. I'm assuming that the main difference between the two is the usual pen style has longer battery life, holds more juice. Are both, mini and pen style, good or do I need to just aim for the pen style? I have a 510 and an Npro that is on the finicky side (sometimes works great, sometimes not so much), so I would like to have a third set to be on the safe side and am looking at the 901's. Any help and or suggestions would be great!