Don't know about durability but sounds like you should just pick one of the available ereaders and invest in a good case to go with it, so when you do eventually drop it, it won't necessarily break.
For pocket sized, the PRS 350 you already had in the official Sony case seems very sturdy to me. Personally I'd probably just buy a used one from ebay and invest in a good case, as that seems likely to be the cheapest option. But there are a couple of fairly cheap options available that have been released fairly recently - neither has sound though:
Both the Kobo Mini and the Trekstor Pyrus Mini are available at under $100. Yyou can buy a Kobo Mini at an Indy Bookstore in the US (
http://www.indiebound.org/ebooks), or online at
Powells for $80. The Trekstor is more readily available in Europe, not sure if you can buy it from their
online store and ship to the US or not, but it's 50€ (about $65).