personally i would say SQL Server, as if your using two products from the same vendor, you should get better results. also if your using web developer it gives you the chance to really use some of sql server to your main advangate as there is a really good gui for interfacing with sql server.
as far as i know there you need to download the oracle data provider to use oracle well with .net, orcaled called theres ODP, this owuld be the same if you used another database that isnt microsoft.
SQL Server already has n sql server libary built in. i havnt used a lot of oracle, but most of the people who i knwo do have said the sql server is easier to configure and use. im going though using orcacle at the moment so ill let u know at a later stage which i prefer.
oracle is a EXPENSIVE, SQL server is a little cheaper. if you want a free database you could look at mysql.
. Net is a programming framework, used for developing window forms/ and web applications