What are your needs? There is good and bad about both. Maybe if you can share with us what you plan we can give you a better answer.

In the meantime:

mySQL Pros:
Free - Unless you're making money from it then you pay per license (i think)
Fast
Multi-Platform
Handles large database well

Cons:
Poor Interface
Limited ANSI-SQL92 Support
No Support for Trans/SP
Clumsy data transformation when using VFP/ODBC
Added maintenance Overhead

MSSQL Pros:
Handles large database well
Easiest "Out the box" Setup
Lower maintinance overhead
Works fairly well with VFP
Supports Trans/SP
Full ANSI-SQL92 Support (and then some)

Cons:
License Fee is and outrage
It's a Microsoft product (<--- Oooops! Did I say that?)
Not Multi-Platform
Can be slow if not maintained correctly
Some VFP/ODBC data transformation problems (but better than mySQL)

Hope this helps...