I have heard that Oracle DBA's can demand a higher salary than MSSQL Server DBA's, however the available jobs for MSSQL Server are abundant. Other benefits of being in the SQL Server world is that our community is unmatched. The SQL Server community goes above and beyond to help each other out, such as this site you are on. We dedicate a tremendous amount of our spare time to volunteer on forums, and community events. There is much more to a career than $$. For me the fun and pleasure I get from the SQL community out weighs making a few extra K per year.
Oracle probably can get you more money, but only if you have the right specialist skills within the Oracle world - I was looking at a job that would have gained me experience of Oracle Financials, and that can land big bucks on the daily rate. But I suspect that the ubermenschen of the SQL Server world can also pretty much demand what they want, and get it. I seem to recall hearing of one organisation that was looking for a SQL Server MCM, and was prepared to offer US$200k or something. They got no takers. I wonder why? (Mind you, I wouldn't have gone for it...)
Some sites do not run SQL Server, since they had to run virus software on the Windows machine...that is - to say the least - not good... So on the Linux platform Oracle, MySQL, postgre or heavy duty databases. Not relational databases could be the future. Money is not all...having fun is more ... fun...
There is a reason Oracle DBAs make more...it is a pain to administer that stinking software. Oracle has so many different modules/pieces to it, which is what causes the high price. Patches are released more often with Oracle, compared to Microsoft. You have to have a strong knowledge of Red Hat, Solaris, etc that Oracle runs on since it integrates so much with the OS. I don't think there is a huge difference in salaries between the two products, it will just depend on the market for the area you live in. I live in Montgomery, AL. This is a miltary town and the militar thrives on Oracle, as does the federal government. Where as when I worked for the State it was a Microsoft shop at most agencies. If you know Oracle in Montgomery you can pretty much make a killing. There are SQL DBA jobs, just fewer of them in most cities. Birmingham area has a good number of SQL jobs and Oracle jobs available. So it just depends on where you are at, or where you want to move. I would suggest though picking one product. The job I am at know is going to make me learn Oracle, which I do not mind, I want to. However I do not expect I will learn Oracle as deeply as I have, and still want, to learn SQL Server. I just don't think my brain could take it :)
Having worked in both areas for some years starting with Oracle and experiencing SQL Server mature, cross training between the to isn't so difficult. I worked in organisations that ran Oracle on Windows OS (don't tell that to an Oracle person, they frown at this). The more onerous bit is often learning the environment differences, unix, linux vs Windows OS. Money-wise as one of your posts says it depends, and generally although Oracle might pull in some more, it isn't earth shatteringly different, certainly on the contract market. What will be interesting will be SQL Servers in roads into the linux world, with powershell and visual studio .net core starting out already.