I would not bet on a new device from Plastic Logic. You have probably read the complaints/requests for improvements concerning the software of the M92. It is already good for technical decouments (I use it on a daily basis for several hours for just that reason) and probably the most advanced piece of software for your purpose. Still, lots of thing can be improved. If Plastic Logic made a product superior to the M92, they would not just need matching hardware specs (probably not a big deal), but also really good software right from the start (huge problem).
In essence, I think it's safe to buy an M92 right away. If you don't mind the extra €, I would recommend you buy at ereader-store.de. The owner of the shop is THE most helpful man with regard to the M92. Plus, he is the only one that seems to be able to make developers at Onyx work on specific tasks to improve the M92. I imagine that by buying there, you support future improvements. Nevertheless, the M92 is great as it is, so you will probably be quite happy for several years.