www.orafaq.com
has some articles about sizing Oracle support for an application. It has more to do with transactions per minute, number of tables, number of simultaneous users, and total data store required. Most Unix Oracle apps are distributed - PC's running Oracle Forms, and a UNIX box with lots of disk space and memory -- all on a fast network.
You should look to support issues first and foremost - like can you get hardware & software support (ie., people) on your site the same day?...
And production versions of MS SQL server will only run on certain editions of Windows. For instance, SQL Server (Standard) is only installable on "Server" editions of Windows only.