I'm not sure about the digital vs non digital but I think it has something to do with temperature control. The only vap I've ever hear of that has this is the vapir and I've heard mixed reviews about it. Apparently the first ones out simply didn't work. Maybe the newer ones are better.

Beyone that there are two major types of vaporizers, convection and conduction. A conduction heats the weed in a metal bowl, usually filling a glass dome with vapor. This is the type that your e-bay link shows. Avoid this type. Yes, they are less expensive but don't work that well. They will get you high but you end up wasting a lot of weed. I had a BC Vaporizer and ended up throwing it away.

The other type, the convection style, draws hot air through the weed when you hit it. The Vapor Bros, Aromazap(the one I use now), and I think the volcano are all this style. They are much more efficient and you only need a small amount of weed to get high. They cost about twice as much as the conduction(glass dome) style but you more than make up for it in the amount of weed you'll save.