Choosing RDBMS is not easy, there are many factors involved such as
1. size of the database
2. number of concurrent users
3. budget for both hardware and RDBMS
4. technical expertise
Access is suitable for small database and is not suited for concurrent use.
sql server and oracle both can work with large database and concurrent users. sql server is cheaper compared to oracle. since sql server only runs on windows platform, you are limited in your os choice. if you are a unix shop then you have to go oracle way.
Also sql server is easier to administer compared to oracle and Microsoft provides many data management tools (such as data transformation services, reporting services, analytical services) for free.
Advantages of Oracle over SQL Server:-
1. More reliable, I've looked after a lot of Oracle databases and most had an uptime measured in hundreds of days unlike most sql server databases I've worked with.
2. Oracle has major advantages in terms of locking and concurrency. In effect Oracle has been designed for the worst case situation of lots of users reading and modifying the same data. SQL Server and Sybase have been designed for the best case situation so in simple benchmarks it usually outperforms Oracle.
3. Lots of scope for tuning in that there are hundreds of tunable parameters. You can also find out a lot about how Oracle works thorough the internal views.
1. Takes longer to learn and not as simple. Less qualified professionals available.
2. Out of the box doesnt perform as well as sqlserver.
3. Cost a bit more if you dont include downtime cost.
Depending upon the configuration you require, pricing for Oracle on Windows can be very competitive with SQL Server. Oracle has entry level pricing for Oracle on Windows systems that support up to two CPUs. For instance, from the oracle store (oraclestore.oracle.com), the list price of a one year term license for Oracle Standard Edition on a single CPU system is $2224. This includes one year of product support and software updates. (The store defaults to choosing perpetual Oracle licensing, so you need to explicitly choose 'Term Licenses' from the left portion of the screen.)
Some Oracle product links that you may find useful follow...