There are 2 sizes of burners, both 12 volt and 120. The smaller chassis burners have smaller fans, limiting the fuel nozzle firing rate.
As far as 120 volt burners go, a power source doesn't care which size you're using...both will draw about the same amperage.
For the 12 volts, there is the ADC (small chassis), and the SDC (large chassis). The ADC will do, at best 1.75 - 2.25 gph firing rate. The larger SDC can go on up to 3.5 gph. The problem is, the ADC can run on any machine with an engine producing 16 - 20 amps charge rate. The SDC requires considerably more electricity to run, so a high amp charging system is necessary. The big 3 manufacturers (Honda, Kohler, and Briggs Vanguard) have high output charging systems on select engines, but these are not standard equipment except on machines set up to run the SDC burner. Kubota engnes with an external, belt driven alternator can run an SDC as well.
The 120 volt burners are more reliable, don't suffer from as many variables acting on them, the components last longer, and are less expensive to buy parts for when the time comes. Plus, you usually end up with an auxiliary power outlet too.
A 120 volt won't heat any faster, but will heat more GPM than an ADC burner.