Backpack vs. Google Sites

It's easy to compare Backpack from 37Signals and Google Sites. They serve the same purposes and offer almost identical functionality. Of course, they accomplish that functionality differently [Backpack is pretty while Sites is simple.], but is one better than the other?

Let's compare the two tools based on 4 factors: user experience, flexibility, aesthetics, and price.
User Experience
Backpack's drag-and-drop nature is fun without sacrificing functionality. Sites' uniform menu structure is simple without sacrificing features. Backpack has cleared marked features (pages, reminders, messages, calendar, and journal) which makes it easy to mentally grasp all that you can do with it. Sites' features are less obvious, especially because the ability to embed gadgets is hidden in a menu. Backpack wins.
Flexibility
Google Sites is extremely flexible, which is why it lost the User Experience comparison. Flexibility tends to make apps more complicated. Backpack is famously inflexible. There is a certain page structure that you must follow because that's how it's built. Sites, on the other hand, has many options for extending, integrating, and customizing your site and pages. Google Sites wins.
Aesthetics
Backpack is just about as pretty as it gets for an information management tool. The available color palettes were carefully chosen and the UI has won awards and been emulated all over the web. Sites is notoriously ugly. White is the predominant "color" and menus rarely win design awards. Google Sites was obviously built by engineers while Backpack was built by designers. Backpack wins.
Price
Backpack has a very limited free version, but to get any real value out of the app you'll need to pay a reasonable monthly subscription. Google Sites is free and nearly unlimited. The only limitation is storage, which isn't a limitation as much as a reason to use other storage solutions like Google Docs (free), Dropbox (mostly free), or Box.net (mostly free). Free beats paid every time. Google Sites wins.
Based on my informal review, it's a tie. With any application there will be give-and-take regarding features, functions, design, and pricing so you'll just have to figure out what's most important to you and then make your decision. [Come to think of it, that's probably how you should make all of your software decisions, web-based or not.]
Both solutions will definitely get the job done for you. If you want cheap and flexible, Google Sites. If you want beautiful and easy-to-use, Backpack.