Is there a big difference between the mk1 and mk2's?
I suspect however that the advice given above in relation to upkeep is very sensible - like elderly cars, you would be better buying on condition rather than spec. I've no idea what the availability of the main drivers is now, but when I had the ones in my mkIIs replaced after one was damaged, I seem to recall being told that there were not too many left. They look the same as the ones in the NBLs, but whether this is correct or indeed helpful, I've no idea. My memory might be playing tricks on me however - this would have been at least 15yrs ago now.
Back to your question - yes, IIRC there is a fair difference. I did once own a both a black pair of mk Is and a pair of walnut mkIIs at the same time and so could try them back to back as it were. I had one pair or the other for the best part of 10yrs and recall them well. I absolutely loved them both.
Over time, I tried them with a number of amps/sources and recall that the mkIIs were definitely better - fuller sounding without losing anything of the many fine qualities of the original and were better able to show the differences on better sources or amplication. Not night and day by any means, but if you can find a pair of mkIIs, they are the ones to go for and I would personally be prepared to pay a fair bit more for them if I were after another pair.
You may have to wait however, whilst I have no idea of the figures, the mkIIs have always been much harder to get hold of and there have never seemed to be too many of them around. At one time, you could have them upgraded at the factory, but this would not I am sure be economic now. I am almost certain I recall being told that there was more to the difference than just the drivers. I think it had something to do with the soft "insulation" material in them changing, but from what to what I could not now say. They may also have been a change to the crossovers, but I might equally have got even more confused and thinking about NBLs here.