Well, the higher the number, the more gain, and more modern the tone.
Quote by Pinky&The Brain

Are you pondering what I'm pondering pinky?

I think so, Brain, but where would we buy rubber pants at this hour?
the 2203 is the 100 watt single-channel jcm800 head, 2204 is the 50 watt single channel head, 2205 is the 2 channel 50 watt head. many seem to like the tone of the single channel jcm 800s over that of the 2 channel, but, as always, tone is subjective. the 50 watt versions are a tiny bit more crank-able so that you can get some nice tube saturated dist...but they are still very very loud.
i have a 2 channel 2210 which is a 100 watt head i love the tone, either way if you want pure tone a 2203 is by far the best choice if you can find an original, so amazing with an OD pedal they can do anything, cleans are easy to obtain just turn down your volume knob and clean up the tone, turn it up when you want distortion, so many possiblities, im drooling while im typing right now
my gear//
ESP Horizon Custom
Gibson LP Custom
Gibson V
Orange Rockverb 50
Mesa Triple Recti
Marshall JCM800 2210