[freecia]

I use both Stanza and Calibre on my mac. Here's a few things off the top of my head in regards to viewing the files, converting them, and having the title/author appear correctly on your eReader

Stanza Desktop for mac
pros:
faster to load than the calibre viewer
exports a variety of files, which is great if you don't mind missing some TOC
Can send ePub files to Stanza on the iPhone/iPod Touch
reads a lot of different eBook file formats all in one app

cons:
Doesn't auto-gen a TOC if none is present
File conversion doesn't always set the metadata (like Title and Author) correctly.

duly noted neutrals for me:
Stanza flows the pages one after another kind of like Word's reading view.
no "send to device" button

Calibre for mac:

pros:
allows for content management including setting metadata
batch convert to one of the supported file types
can auto-generate a Table Of Contents if the file has the right tags or if you're familiar with Regular Expressions

cons:
batch convert is only to 3 file types.
No Find and Replace for batch editing of metadata
Need to set the title/author guessing to the correct format before you import especially if you have a lot of books or you'll be editing the titles for a while...

duly noted:
viewer is more like a browser viewer or Word's print layout view.