Wow, your display solution is really a sight to behold! Placing all 17 helis on one level really has that squadron presence! 
The Detolf is really fragile. It doesnt use tempered glass too unlike the Billy, so please be warned especially if there are kids around the house. No doubt the Billy bookcase is really appealing, but due to budget limitations, i opted for the Detolf. Also, i find that the Detolf has that 'display appeal' due to the all-glass surround as compared to the Billy. Ended up using my sole Billy as, well, a bookcase as it was intended 
Well i think in terms of stability, both are more or less the same since i secure them to the wall using 'U' brackets. However, like you said, it's the all-glass construction that's worrying!  As hobby budget allocations are almost always the least and the most 'sacrificeable' of all budgets, i ended up with Detolf(s) instead. Only ever bought one Billy, used it as a bookcase instead in the end since the other Detolf(s) line up more uniformly as a display unit