It is unlikely that SQL Server is going to work any better for you
than MySql. Capacity could perhaps only really become an issue when
you start storing image and video data and other large blobs.

If you have a lot of money you might want to look at Oracle with raw
partitions. I don't know if any other database systems can use a raw
harddrive partition directly without the filesystem in between. Anyone?

You should also consider PostgreSql, which allegedly performs better
under high load with high data quantities.