I think, thermostatic showers have a kind of sealed valve inside which regulates the temperature by opening and shutting the cold and hot flow. Digital showers must use electrickery to achieve a similar result.
I installed a thermostatic shower recently all on my little own so it can't be that hard to do. Works like a dream. I must say the instructions looked daunting but when you got down to it it was dead simple.
There is not actually that much difference between them, they both do the same thing in much the same way, the only difference is the water temp on a digital one is controlled by a little 'puter and electronic valves, on a thermostatic one the water temp is controlled by manual valves operated by a manual thermostat and springs.
the digital one will probably look flashier and may have a digital display to set the water temp and will cost about twice as much.
thank you Chuckie. is the digital one more likely to develop faults? and it needs to be connected to the electricity, I assume? (that was a stupid one, huh?)