Both are great drams that are simply adorable and dangerously quaffable. Around the time of their release they were quite affordable for a 40-years old and still a price point of around 500 Euro is okay given the quality of these drams. If you can afford it go for it at auctions (but remember there are many later releases of a 40-year Glenfarclas that might be different to these)...
The 2010 has a colour of mahogany and the 2012 is around one shade paler at tawny. The texture of the 2010 shows very late sticky tears and late slow legs. The 2012 shows some earlier and a little faster legs but still very late fat sticky tears. The 2012 offers a very bold and complex nose on fine herbal sherry aromas wih some spicy wooden notes. The 2010 owns less a herbal but a more sweet and fruity sherry profile that is at least as complex as the 2010. So the winner depends on your preferred profile - I like the more fruity profile of the 2010 a little better than the herbal of the 2012 but this is just a nuance.
The taste of both drams are multi-layered and tackle the taste buds with an unbelievable array of different flavours. The 2012 is nicely balanced on all the sherry flavours one can imagine plus some very nice spices (even some salt) and herbs. The 2010 is a little more spicier (peppery) but more chocolatey too. Hard to decide which one is better, in case of doubt I would prefer the 2012 here (but again it is just a nuance). Some water opens the nose of the 2010 even further but flattens both the taste and finish. Same with the 2012 so nose it with some water added and drink it neat (You think, that is impossible? ...simply use two glasses).
The 2010 immediately coats all of the mouth with a warming feeling, no distracting moments to mention (the tannins are just perfect for my taste). Same with the 2012 that is even a little more charming on that very first impressions (maybe it had the advantage that the 2010 already coated the mouth before). The finish is unbelievably long on both drams with no distracting bitter or drying moments. The 2010 vanishes nicely in different waves that get more and more chocolatey over time, really adorable. The 2012 gets more herbal again that is a perfect contrast to the sweet sherry aromas. Nevertheless I like the chocolatey finish of 2010 a little better.