My preference is for ePub. The size of images in a .pdb (eReader) format file is usually smaller than that of ePub, and in my experience, images in .pdb are targeted for smaller devices like 240x320, and images for .epub are targeted for larger devices. In general, looking at an image from a .pdb on my computer or even the nook looks blocky and ugly.
Having said that, I have a huge number of .pdb files purchased at Fictionwise because of the 5% additional Micropay rebate, and .epub files from Fictionwise can only be downloaded on Windows/Mac because they have to be downloaded with Adobe Digital Editions. However, if I know or expect that a book has illustrations, I'll go for the .epub.