First the basic answer:
An analogue trim is an actual lever on a ratchet that you slide to one side or the other to obtain the desired trim.
A ditital trim is just a rocker switch, as you move the switch to one side or the other you will hear 'beeps' from the Tx to indicate movement and there should be a graph on the Tx LCD to indicate the position of the trim.

Before the advent of computer radios, the norm was to adjust the pushrods on the plane after each flight so that the trim levers could be returned to the centre position. You could easily knock the trim lever and not remember where it should be. Also, if you use the Tx for multiple planes, the trim would be different for each plane unless you made the appropriate adjustments to each plane after every flight.

I recently got a 6XAS (it was being sold off at the LHS) and it is an older design radio with analogue trims, so the original radios probably had the problem described above as the TX could be used with 6 planes. Futaba probably added the software at a later date to store the trim positions. You have to go into a menu to do this and then set the actual trim lever back to neutral. Not really an ideal solution.

With digital trims on more modern radios, the trim positions are automatically stored when you make any changes, so there are no problems with the lever getting kocked or the position not being saved. The trim will be correct for each model if you switch planes.

The 6XAS is still more expensive than the 6EXA because it offers more features. I don't know the specifics but the 6XAS offers PCM and programming for heli's for example.

For the control surfaces, I think the digital trims are a better solution, and certainly better than the half-ar**d bash Futaba have done with the 6XAS.

Digital or analogue trims on the throttle are a whole new subject.

I've got the XP8103 which is also an old design, but at least JR updated the trims to fully digital with the exception of the throttle. I used to have a digital throttle on my XF631 with a throttle cut button, but I like having the analogue throttle trim on the XP8103...