I can't put into words how highly I reccomend the Huntsman four sensor
unit, part number HUT1105VXL over at the AirGas site.
It's very expensive but I've never handled anything better. I've gone
through a number of auto-darkening setups and two things always
bothered me: The small window and the two sensors getting blocked. When
you do any out of position welding the two sensors get blocked all the
time which means you get blinded for a few seconds and lose your place.
This one has a very large window but more importantly four sensors, one
at each corner of the unit, which makes it almost impossible to block
all of them. Stick it in a nice light helmet and you're set.