Hi mate - though a died in the wool Nicolai Fanboi and long time rider of several Nic's I havne't ridden the Ion 16 and so can't comment directly on that. In fact I am not sure anyone in the UK has one as yet though there are one or two on order and due any day now.
What I can tell you is that you're comparing chalk and cheese by comparing Banshee and Nicolai. It's like comparing a Scooby Imprezza with a Porsche 911. Sure they both have the same performance on paper but if you're in the market for a Porker, you're not also in the market for a Scooby!
With Nicolai, you will have superlative build quality and longevity. The big gain in their attention to detail is accuracy, tangibly in alignment - which means the things don't wear out or get lose in a way that can't be adjusted (the bushings are designed to bed in and have adjustment screws to remove play) - and subsequently in laser like tracking and handling.
The frame is made specifically for you, i.e. they aren't sold from stock, so you get to specify little details and chose the colour but it also means you have to wait (and the wait can be as long as three months in some instances) but the wait is worth it.
The Ion 20 I owned rides in a way that they tried to replicate with the Ion 16. When I spoke to them about the Ion 16, it was stll in development and wouldn't be ready until summer this year so when I ordered my last frame, I went with another Helius AM but with full custom geometry. When we talked, I described how the Ion 20 had handled like a DH bike but pedaled like an over grown trail bike. This is what they said they were aiming for with the Ion 16.
So if it's anything like the other Nic's I've owned it will be very stiff, long front centre with a relatively short rear (very like Specialized) and with a suspension feel that is best described as very neutral and transparent, i.e. it has no discernable character in the way say a VPP or single pivot bike does. It's plush over small chatter and big rocks, but it's not quite as good when the wheel falls into holes or over small steps.
The Banshee is mass produced and may be a great bike but I wouldn't know. I do know that it's in no way a substitute product for a Nicolai. The quality isn't remotely comparable, although the performance may well be.
While not handmade in Germany, you should also take a look at Liteville, either the 301 or the longer travel 601. The former gives as much travel as the Ion but weighs about 30% less (which I couldn't get my head around and in the end was a major psychological block to buying one) the former weighs about the same but has way more travel in it's top setting.
Whatever you do have fun deciding. If you want to try a Helius AM that has been built with identical geometry and sizing as a medium Ion 16 let me know as I would be happy to meet up and let you try mine.