There are advantages/disadvantages to each. Aging in bulk will minimize the oxidation that will occur (all other things being equal) as it has less surface area. Some level of oxidation can be considered a benefit in a barleywine, but it can go too far.
So I think if you are looking at keeping a barleywine for a very long time, it is best to do the primary aging in the fermenter, assuming it is well sealed (air lock) or in a keg (which also offers you the easy ability to sample it at different ages). Bottles are an advantage as it will free up a fermenter and potentially make storage easier (as well as make sample it at different ages easy, also).
A couple other things to consider, you're going to lose some carbonation on bottles, especially if you keep them for a long time. Corking can help this, but again, bottling after the primary aging works well, also. And a keg or a carboy is going to have more thermal inertia than individual bottles so will be less likely to suffer from temperature changes.