Should I buy a Dahon or wait for a Tern folding bike?
I think that the only possible answer for this is to wait and see. If I needed to buy a folder now, I'd stay away from Tern until they became a more established company. While it's possible they may take over the market and distribution channels, it's also possible that this new company may fold (no pun intended). 

Dahon seems a good bet for the near-term. Just supposing that Dahon stops distributing all spares tomorrow. If nothing else, there are many Dahon spares already in the wild, and will be available for puchase on eBay if nowhere else. Tern is a new company and doesn't even have that. (This is, of course, discounting the possibility of Tern and Dahon using any of the same parts.) 

If you want a Dahon, I'd go ahead and buy one. The company's not going anywhere just now. If the Tern bikes look sweet, when they become available you can pick one up. Just remind yourself that they're a new company and spares may or may not be available. And lastly, if you want to stay away from the whole mess, you have other options for folding bike brands, as has been pointed out. However, you're pointed out that your preference is to stay "in the family". In that case, is upgrading your current folder an option?  

(This question can't really be answered satisfactorily either way, and I hope that in time someone else will write better answer as more information becomes available on the Tern/Dahon rivalry.)