Hello Claude!
I'd say that either of them would be an excellent choice.
I don't think the images should be much (any?) different at all. Big thing that 600D got over the 100D I'd say is the flipscreen which can be useful but I would take the 100D due to weight (407g vs 560g with battery and memorycard) and formfactor (I intended to have a potentially portable rig though so that is my preference).
I don't think the prices are much different (atleast they are not here where I am) and the 100D comes with the STM kit lens which is supposed to be quite good.
Best, Magnus.