If you want to stick to MOBI, then a Cybook or Hanlin are obvious options (in the US, buy Cybook from Books on Board and the Hanlin is called the EZ Reader
). The Cybook has better MOBI support, but the Hanlin is catching up.
If you don't mind stripping DRM (widely done but generally thought to be illegal in the US and several other countries, even for personal use), then there are more options:
a) Buy a Kindle and strip DRM both ways (Amazon AZW to MOBI for your MOBI devices, DRMed MOBI to DRM-free MOBI for the Kindle). Amazon obviously wants to lock in Kindle users, but in my view there is a premium built into the Kindle's price that goes towards offsetting the price of Kindle ebooks. This premium is more palatable if the Kindle ebooks are readable on other devices. Note that a few Kindle ebooks are in the TOPAZ format, which really is Kindle specific.
b) Stick with the PRS-505, strip the DRM from MOBI and convert to LRF or ePub using Calibre. MOBI can still be your primary ebook format, but there are some ebooks that are only available with Adobe DRM (PDF or ePub) and you can buy these (or check them out from a lending library) as PRS-505 only ebooks.
Note that the PRS-700 is the first in a wave of new 6" ebook readers that will hit this winter/spring. We already know that there is a Kindle 2 in the wings, and also the EZ Reader plus/Pro. There will presumably be a new reader (or readers) from Bookeen as well. These devices will have better screen controllers and faster processors. This may or may not lead to better ebook readers, but it will likely freeze all software development on older models. So don't buy a device expecting much in the way of software upgrades. This argues against the Hanlin, although I think its MOBI support is already "good enough" for many readers and improvements to its Java MOBI Reader may be equally applicable to its new and old devices.