I own both ZBB and AXPW.OB, but gotta say AXPW is making the most obvious progress recently.
I personally think it's a screaming buy, but it's also equally possible I've been deluded :-) And yes, it does have an official "going concern" in the SEC filings. I think that's a mostly CYA, but it is what it is. Total absolute speculation.
If you've got the time, there's tons of info out there on them ... we're either cultists, or the most dedicated set of small stock holders you're ever likely to see.
Both ZBB and AXPW offer new innovative products for the clean tech storage area. ZBB's product is more expensive but meets a different need. The ZBB electronics are the best at attaching different products to one energy storage device. Don't be surprised to find ZBB electronics with an Axion UPS system sometime in the future.
Both are on similar timelines although ZBB was first to the production table.