Yeah, a field recorder is a bit like a camera in that sense - the best one is the one that you have when the moment strikes.
Re-reading the original post though, I’d throw some support at the Sonoma Wire Works Guitar Jack interface. It seems to be the best-cheapest-smallest way to get stereo line in and out for an iDevice. If you have to buy one of those to go with it, then maybe something like a Zoom h4 makes more sense, but if you have an old iPhone or iPod touch that needs a new purpose, it’s a pretty reasonable bet.