I only have experience on 646, but the difference between it and the 645 seems largely apparent in specs (number of cuts, slits, etc per sheet) and I do not believe the feeding is as good.
Of course a 645 will be older and will have more wear on the knife / slitters as well.
I don’t use any templates and don’t have the Fiery impose to make setups, i make all of our programs on a computer running the duplo software. I can’t imagine programming it from the control panel. I am a big fan of the machine, it is easy enough to setup if you are good with a ruler and if your printer has good registration. If you have any registration or static issues it can be a headache. Ours has the side air blower which I believe was a later 646 retrofit, may make your life difficult by not having this if you have any kind of static or thinner stock.