As a developer who codes both asp and sql I've found that the answer usually lies with which ever makes the client happier.  If clean asp code is the clients main goal then the "null fix" is coded in the proc.  Speed here, like Andy says is pretty much a mute point.

Personally, I found it made my DBA happier when I used "the power of SQL" and the client really could care less how the data was handled, provided they received the results they wanted.  I'll go further to say that a Good DBA is a Happy DBA and anything I can do to help keep my DBA in a good mood is in my best interest, and thereby in my clients best interest.

I like my code clean, both in ASP and SQL.  For me the SQL answer provides the the cleanest solution, less code, easier to maintain and more reusable.

That's my $.02!