One DB machine for Two WebSites

When there is 1 DB machine for 2 web sites, is it better to have

One DataBase-instance running with 2 DataBase-Initial-Catalogs ?

or

Two DataBase-instances running, each with 1 DataBase-Initial-Catalog ?

using SQLExpress or SQLServer

please only look at those possible, simple conditions.

forget about read/write same tables, security issues..

Interms of Memory/CPU/ Performance, which setting seems better ?

Please give some thoughts on Performance / DataLoad / Consumption,,,etc...

thanks.

For pure performance and simplicity, one instance of SQL Server with two databases will work better. With multiple instances of SQL Server, your available memory is split between the instances.|||

Thanks.

But if another instance is running and loads in memory,

would it be faster for getting results in database ?

Assume I have lots of memory.

thanks.

|||One instance will be faster than two instances, since the single instance will have access to all of the machine's resources (RAM, CPU, I/O).