Between those two, I'd definitely go with the Delica 4. Personally, I would recommend making sure that if you buy a Delica, that it is in fact the Delica 4, though. The earlier Delicas are very good, but the Delica 4 is great. I love every change they made, and love this knife.
That said, if you haven't looked at it yet, you ought to check out the Spyderco Salt 1, too. It's very similar to the Delica (in fact, the blade shape of the Delica 4 is about halfway between the blade shapes of the Delica 3 and the Salt 1). It uses H1 steel, which is so close to being completely rust-proof, that you never have to worry about it. Makes for a very low maintenance knife. I've been carrying one as my main EDC for two years now.