The difference is the size humidor you will be humidifying. The Cigar Oasis EXCEL is made for desktop humidors up to 4 cu. ft. or approximately 400 cigars. The Cigar Oasis PLUS is made for larger capacity boxes over 4 cu. ft. and up to 10 cu. ft. Although physically the two models look the same from the outside, the PLUS has two additional air ports in the rear of the unit and an additional hole in the cartridge. They allow the PLUS unit to draw in more air and push out a large amount of moisture for larger spaces.