A few ways of looking at this really ....
I think in the short term the single cask (OB only though) will appreciate much more quickly due to the extreme limited availability in comparison to a Limited edition which these days seems to be in the 1000's
Of course what Willie says is spot on .... a bottle with some renown will be a very good investment (if your that way inclined) when it is long gone .... and as good if not better than a single cask.
However then you have these runs of single cask which seem to come out every year and I think that just takes the shine off it and I for one think that's a good thing as all these single casks are trying to exploit the collectors market and once the Distilleries over milk it then some of the madness may stop
I'll hold my breath though