I use an EXTECH clamp meter that has a 2000 amp (AC/DC) range, voltmeter, ohmeter and 2000 degree F pyrometer.
I used to own several AMPROBE meters and one Fluke, the EXTECH was by far more accurate at calibration time than either one of the other two. Went through two AMPROBE's in one year as they are not very robust instruments - at $300 a pop plus the $50 calibration charge.
I checked into the Miller DM unit, it requires 120V power supply and has to be hard-wired into the welding circuit (e.g.: twist lock welding connectors). It does not use inductance to measure amperage.
EXTECH also sells their meter with a NIST certificate. All that, plus the acurracy, not bad for $250.00
www.extech.com
p.s.: This is an averaging-type meter, there are some fluctuations with changes in arc length, stick out, etc., but not like in my friends true-RMS meter which you also have to do some math to determine the average amperage.