A few years ago, I would have said aircraft was harder to build. This opinion was based on my experience with older aircraft kits. I wouldn't even build aircraft kits in my previous modeling heydays of the mid- late-'80s. It was armor all the way. Very forgiving in the finish department and regardless of the kit, I have always found them easy to build.
Aircraft, on the other hand, have seams all over. Smooth flowing lines that have to be perfect in order that they don't show up like a sore thumb once painted. Making sure dihedral is correct. Making sure stabilizers are lined up properly and symmetrical... the list could go on.
With more modern kits, though, I find that aircraft kits are much easier to deal with. But, at the end of the day, considering all aspects of a model project, I will still have to lean more towards armor as being the easier of the two.
Maybe that's why I have been building more aircraft kits lately.... trying to improve.