I'm down to my last bottle of 5%, so I thought I'd do one more A/B comparison. They're almost 6 months old now, so I thought I'd check in on how they've aged.
Aroma - 3X slightly more "floral" hops-scented, 5% had a more distinct malt aroma (like a vienna lager). Both had very little aroma.
Appearance - 3X formed about an inch of dense white head, which lingered throughout the glass. 5% formed about 1mm of head, which mostly subsided. Clarity was outstanding on both. You could read a newspaper through either, but 3X appeared slightly clearer, possibly because of the lighter color. 3X had slightly better lacing, but the difference was minimal.
Flavor - The hop flavor from 3X was more floral, while 5% was more herbal. 5% seemed both more sweet and more bitter; more sweet initially then more bitter in the finish. Maybe related to the herbal character of the hops? 3X had a more subtle, but nuanced flavor. 5% was more "one-dimensional."
Mouthfeel - 3X is noticeably drier, with perceptible, but not obnoxious, astringency. The mouthfeel on 3X seemed more full. Carbonation levels seemed close, maybe the same, but I'm not great at perceiving small differences in carbonation.
Overall - I think 3X has aged more gracefully. 3X is better, and 5% is worse, than I remembered them being. But the differences weren't pronounced enough to fundamentally change the perception of the beers. The subtle herbaceous character that developed in 5% is definitely interesting, and I'm not sure what's going on there.
At 6 months, I'd say 3X is a slightly better beer. The hop character is fresher, the mouthfeel is fuller, and the flavor is more balanced. There are so many variables I won't say, definitively, that decoction increases the cellar-ability of a beer, or anything like that, but it's interesting to see how the beers have evolved. I'm acutely aware of the possibility that "operator error" could be responsible for all of the differences.