I did a test today, coincidentally, to see which would charge faster, the WFCO or a Schumacher 12 amp charger. I left my two 6 volt batteries off charge for a couple of days so I'd have something to compare.
The Schumacher charged at about 8.5 amps initially and the WFCO was at about 6.5 amps. Within a few minutes the WFCO was down to 1/3 of an amp @ 13.2 volts. After connecting the old Schumacher, the charge was 2.5 amps @ 14.5 volts. This all occurred in about 15 mins as my batteries were no discharged much. However, in my experiment, the 12 amp Schumacher will charge faster than the WFCO. If the batteries were down enough to take over 12 amps, the WFCO would probably do a better job till I could put the Schmacher on to top them off.
Rockwood 2104S, 2014 Ram 2500 Diesel.
USMC 68 -70