The XL Plus is for larger humidors. Although they look the same the difference is in the way the air is routed through the unit.

I have the XL plus and the XL II. They work perfectly as long as you live in a fairly dry climate and you have zero faith in the accuracy of the humidity settings. I used a digital hydrometer to get an idea of where the unit should be set initially then made small adjustments over a couple of months until the cigars were just right for me. I haven't had to touch them in a couple of years, although the XL Plus has to be set at 71.5% to maintain about 65% RH.