SQL Server is only on Windows. You can get Sybase ASE for Unix and they are somewhat similar to SQL Server.
I have nothing good to say about DB2 for unix. We had all kinds of problems with version 8.1 ESE. Seems all our other products did not have proper drivers for the database. Cognos, Ascential, Evoke, etc. DB2 for unix is lower on my list than Oracle, Informix (which IBM owns), SQL Server, Sybase, and Ingres (CA).
A key difference; stored procedures for DB2/UX are compiled into C code and are not transportable in case you want to get away from IBM to another RDBMS later on (Oracle, SS). You will need to buy a C compiler if you choose DB2 for unix.
SQL Server is miles ahead as far as database administration. Check out the cost for a DBA for DB2 versus for SQL Server - about half the price. Furthermore, SQL Server come bundled with Replication (extra charge for DB2), OLAP (extra charge for DB2), DTS (Warehouse Manager available at extra charge), and Reporting Services (no equivalent from IBM).
SQL Server is scalable to the low Terabytes. I just finished a gig with a 3 TB SQL Server datamart.
I was on a project at one company where they pulled the plug on DB2 8.1 Unix in favor of SQL Server after wasting several hundred thousand dollars. I am not at liberty to name the company, but I can give you a reference who might.
DB2 for Unix - DON'T
Sometimes the grass is greener on the other side because there is more manure there - original.