This isn't really a "how do I" question, but a number of people asked me to compare Dropbox to Box.com. (Who was giving away 50gb of storage around the release of iPhone4s and iCloud.)
Here's some quick comparisons:
Apps for iPhone, iPad - BOTH
Folder syncing apps for Windows & Mac - Dropbox (Box.com has a Windows version if you upgrade)
Free cloud storage - 50 GB for Box.com (100 MB upload max) 2 GB for Dropbox
Sharing files and folders - BOTH
Open API for developers - Box.com
Google Apps integration - Box.com
My recommendation: I have both accounts but only really use Dropbox. Box.com is just one upgrade opportunity after another. With Dropbox I've never felt like I need to upgrade to a paid account. (I've maxed out my referral link at 10 GB)
The reality is that people like simple and Dropbox is simple while Box.com isn't. Dropbox just works... no configuration or anything. Just sign up for an account and start using it. Box.com... I've fiddled with it for a while and can't figure it out fully. All I know is that everything I want to do results in me seeing the paywall and that stinks.