I teach faceting at William Holland School of Lapidary Arts and consequently have much experience with most faceting machines. Of the common machines in use today my least favorite is the Facetron. There are so many things that will frustrate you if you get one that I would discourage you from looking in that direction.
Of the two machines you mentioned, I would suggest you consider the Mark 5 over the Facetron. You are already familiar with how it works and all you would have to do is get used to the Digital Display. But the Digital Display has some idiosyncrasies that you need to be aware of. First it reads the angle every second or so, which can get frustrating whey you are trying to approach a meet point. It is not constant reading. Also, it is battery powered and when the battery charge gets below 50%, it starts giving you erroneous information.
If you are looking to upgrade I strongly recommend that you save your pennies and plan to buy an Ultra Tec. You can upgrade to the Digital Angle Dial at a later date. Ultra Tec is by far the most accurate machine out there. Yes it is also the most expensive, but you can always buy one used and then upgrade it to Digital. This is the only machine that I would recommend you purchase used. Please stay away from any other used machines. With any other used machine you may be just inheriting someone elses headaches.
Best of Luck.