Performance wise They are both 28kW boilers that will deliver the same 10l/m hot water at a rise of 39*C with the same Centeral Heating output. The only difference is in the dimension of the appliance itself. The junior been only 600mm x 440mm x 325mm quite a compact little unit that will fit inside kitchen cabinet. Compared to the larger casing of 850mm x 450mm x 360mm of the other boiler.