To be honest, both casks were pretty dead when filled with Ben Nevis new make back in 1996 and they never ever should have been bottled as a single cask release. They most probably would have done their job in a huge batch of blended whisky and I wonder why bottlers like TWC release such stuff. I guess there are simply no other old whisky casks available anymore (due to the long-lasting recent whisky boom). Do you see why the quality degrades during boom times, too?
The sherry is quite pale for a 20-years maturation at jonquiripe corn and the bourbon is at white wine (what is unusual light for such a long maturation, too). I am afraid that both casks were leached and almost dead. The texture of the bourbon shows slow legs and late fat sticky tears (not bad!). The sherry shows faster legs and smaller sticky tears so the oiliness of the bourbon is better. The bourbon offers a very sugary nosing profile with both barley and fruit sugars but I cannot find any aroma that comes from the wood. It is a little punchy in the nose but not as much as the sherry is. I do not find a lot of aromas in the sherry at all besides the pure alcohol. Another sign of a dead cask. Actually, I do not like both noses but the bourbon is a little better than the sherry.
The taste of the sherry is as dead as was the nose - I call this sugared alcohol with far away hints of something that was a sherry cask long ago. The bourbon is sugared alcohol too but at least some notes of fruits shine through. Again, I do not like both but in doubt I prefer the bourbon over the sherry. Water releases some more sherry flavours in the nose and on the palate but still I would not call this a sherry matured dram (but it raises the score by about two points on both dimensions and the finish). Same with the bourbon but this one gets flatter, unfortunately.
The bourbon is nicely coating with some minor hot moments. The sherry is less coating and hotter in its mouthfeel and it has a minor drying feeling (plastic). The bourbon is much better here than the sherry. The sherry finish is short and hot with again some drying notes (plastic). The finish of the bourbon is a little longer and shows no distracting moments but no new flavours either, unfortunately. The bourbon is much better than the sherry here but still on a rather weak level.