Anything you can do in the database is typically more robust. If you handle default values which will be used if no value has been specified only in your app, and someone manages to connect to your database some other way than via your app (and believe me - the users WILL try to connect using Excel or other tools) - then the database is wide open and the user could potentially insert crappy data.
Same goes for referential integrity and check constraints. I believe you're better off if you try to have as many of those constraints on the database - then no matter how the user connects, if he doesn't send you anything, you can put in sensible defaults.
Don't let the application handle your checks - leave that to the database!
Plus it makes your SQL insert statement a lot leaner and meaner if you don't have to specify all the "obvious" defaults, like "getdate()" for a "LastChangedOn" date column etc.
