oracle versus sql server

This will probably turn into a Ford vs. Chevy argument.  In my opinion either one can be tuned to run and perform well.  I think it's more about which are you more familiar with, which one is your IT department more comfortable supporting, and do you already own licensing for either vendor?  One thing to keep in mind from a Windchill perspective, PTC provides a tool to migrate from Oracle to SQL Server.  They do not provide any tool to go the other direction.

In my previous job, we did an extensive research on this during the design of a Global Windchill System to support around ~8K user base.

Here is the summary of what we  found

1. Installation and Setup - No difference in efforts if its monolithic. PSI is supported for both. But for SQL Server cluster, we had to manually create database using SQL Server Management studio.  The clustering technology in Oracle -RAC  was much better and reliable than what SQL server offered on it  2012 version. If you plan to run DB on a non Windows platform, then Oracle is the best option. SQL Server on Linux is a not a matured technology platform.
2. Performance and Stability  - With the help of some skilled DBAs , the general application performance was right where we wanted to be, however we had several issues with SQL server during the initial data migration effort. The transaction log management system gave us a hard time, with transaction logs turned on the data migration process took almost 3x time to complete, with the logs filling up the disk, the database crashed hard sever times. Often we had to reboot the server itself to release the memory. So far in our experience, Oracle on  Linux platform offers the best performance and stability, but it requires more administrative efforts and skill sets to setup.
3. Licensing -  SQL Server used to have a virtual server friendly licensing till 2012 where as Oracle required you license the entire physical server even though you run DB on a server instance. If you have a legacy licensing for SQL server, it will definitely a cheaper option. I hope at some point PTC will support one of the new generation high performance databases - PostgreSQL and so on.
4. Support - PTC support for SQL server is much better than how it was back in Windchill 9.1 days. From an administration perspective, Microsoft has better UI based tools like SSMS, an application administrator can easily manage a SQL Server database with some online help.

Another consideration on your database vendor of choice should be the risk of receiving an "unfavourable" Oracle licensing audit. We have just recently had the "benefit" of a compliance audit (of our circa £50K estate of Oracle) and we were presented with a bill of £300K to pay within 30 days or face sanction !!!

It took us about a 4 man weeks of work (and numerous conference calls) to gather enough evidence to get them to accept that we were working within the terms of the OLSA and for them to finally withdraw the claim. I was worried at one point that my company was going to roll over and have to pay the bill, but we managed to fend them off eventually.

Needless to say, I have successfully migrated my TEST Windchill System to MS SQL Server and our PROD System will be switched in the coming month or so - the performance looks like it will not be impacted. This, along with re-archtecting my remote DR System to use Veeam Replication rather than Application level replication means that I will have totally eradicated Oracle from the company by the summer.

There is no way that I will ever be a customer of Oracle again.