Just to start off with a clear air I will admit right now that I am a diehard Vienna fan and not big on NNWL. My personal preferences aside, let the match begin!
Interface
Many people don't like Vienna's interface but I personally find it acceptable. For those of you who don't know NNW (but not Lite) recently received a interface lift. Let's take a look at the two apps now..jpg)
My biggest issue with NNWL is its interface. I can't pin down a single part 
of it that annoys me but I find the whole package to be rather off-putting. However, this is purely a visual thing, I think the interface is perfectly navigatable and consistent with other OS apps. I think for someone who is less of an aesthete the NNWL interface will do fine. (The interface is also much improved in the new version of NNW). One last little minor thing that I don't like about the NNW(L) environment is the non-standard dock badge (which you can judge for yourself). 
As I said earlier, many people take issue with the Vienna interface. Two issues that tend to repeat are the brushed metal and its inconsistencies with the standard OS X environment (namely, the controls at the bottom and the thick window border). Strangely, for all of my love of consistency these things don't bug me. This may be because I like my news-reader to be more of a widgetesque app (a simple program that is is simple and streamlined). One option that people like about Vienna is the three-column view (shown) that is not available in NNWL. Yet another nice thing about Vienna is the option to use different styles.
NewNetWire Lite: 7/10 - An intuitive but less-than-gorgeous interface.
Vienna: 8/10 - A controversial interface that I feel is diverse and effective.
Features
Now, even though I'm not big on bloated feature-full RSS readers I know some people are. I'm sorry to say it but Vienna wins out here too. Although free, Vienna has many of the features that are in the full version of NNW but absent from the Lite one (these include an integrated browser and a bunch of other stuff). Of course, you could shell out $30 for a couple more features and an interface you may find more appealing but unless you have a hedge fund to burn I'd suggest Vienna for features.
NewsNetWire Lite: 7/10 - Has the features I need but not enough for everyone.
Vienna: 9/10 - The developer(s) could make good money with features like these.
Overall
Vienna has one with a fairly slim lead. I feel that this is one that is really up to the user. It depends what you are looking for in an reader. Please share you opinions.
Vienna - Free
NewsNetWire (Lite) - Lite is free, $29.95 for the full version.
P.S. Here is a screenshot of how my Vienna looks, in case you're interested.
P.P.S Ignore any misspellings...it's complicated.
I have finally caved to HitTail's requests and now I am launching a new feature: Software Sumo! Software Sumo will evaluate two Mac softwares (often freeware...in which case I guess it would be Freeware Fencing). Today we will be kicking off with MacJournal vs. Journler. Soon I hope to evaluate NetNewsWire (Lite) and Vienna. Any other requests you have for match-ups can be emailed to me. And now your regularly scheduled programming....
As the names imply MacJournal and Journler are both journaling applications. For the purposes of fair evaluation I have chosen to match up the latest free version of MacJournal (2.6) against Journler.
Interface
In this first screenshot we see a comparison of the toolbars:
Journler has a nice unified look going while MacJournal has a split aqua thing going on (however, in the most recent, fairly pricey version they have moved on to unified). The buttons are fairly similar but I prefer the ones in Journler.
Next, lets look at the MacJournal window:
We see there are two drawers, one that holds journals and another that holds entries. In the middle there is a basic text entry area with a title box, a main body area and a small button to lock the entry. Pretty boring stuff especially compared to this next shot of the Journler window:
Here we see a three pane, Mail-style layout. On the side we see the Journals (as well as a nifty calendar), on the top pane we see entries (with plenty of information about them) and on the bottom we have the text. The text pane allows for a title and the body text as well as tags, a category and the date. There is also a handy word count ribbon at the bottom of the window.
I certainly prefer the interface of Journler because it is more compact without drawers hanging out all over the place, it provides more data and it just looks better (if not perfectly integrated with the OS's interface).
MacJournal: 5/10 - Looks like it stepped out of Jaguar or Panther.
Journler: 8/10 - Doesn't look completely at home among other windows but fairly nice looking.
Features
Both applications have rich text entry and picture capabilities as well as journal locking and statistics and blogging (which we'll look at in the next section). In addition to these Journler has labels (arrived in a later version of MacJournal) and better media integration. I personally never use the media features but I'm sure people will find them handy. In my opinion, as long as the application has basic text and organization features it can pass as a decent journaling application (or at least an improvement from using MS Word).
MacJournal: 7/10 - Has the basic features but not much else.
Journler: 9/10 - The only feature I wish it had is new Blogger support.
Blogging
MacJournal has full support for LiveJournal and Blogger, Journler has support for most every blogging service. Journler clearly says that it does not yet support the new Blogger, MacJournal does not address the issue but I assume that is the case because it would not configure correctly (this is a problem I have had with all blogging clients besides MarsEdit and Flock). Obviously I cannot try out the blogging features (at least not until they get new Blogger support).
Overall
Overall, Journler has scored better with 17/20 as opposed to MacJournal's 12/20. This certainly reflects my experiences, anyone else who has used these apps want to chime in?
Journler - Free ($10-20 Donation Suggested)
MacJournal - 2.6 is Free, Others $29.95
As you can see, at the moment applications are only being evaluated based on two main points, if you have more suggestions please email them in.