For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
Not really much difference, except the D is "nicer" with the digital controls. Knobs work just fine, and you won't notice any significant difference between a board cleaned in one versus the other. Heat + time + sweeping frequency + cavitation = cleaned board.
The D is for people who are afraid their employees will break the knob on the HT version. If you do, you just have to remember that the board is in there so you don't ultrasonic it for 40 minutes. I don't think it is worth the extra money for the simple reason that you have to hit a select button to change from time/temperature/UC strength rather than immediately adjust dials, and you also have to wait 2 seconds when you turn it on for it to come on. It's very 1980s alarm clock, and a pain in the ass.
We sell them all now. We became a distributor of Crest: you can buy here.