I started with Java, Switched to C#, and then switched back to Java again!
I like Java technologies better as there is more choice, more open-source software to utilize and support and a great community. As far as .NET is considered, I will admit that C# is a superior language and that Visual Studio is a GREAT IDE.
With that being said having EXPERIENCE in both looks impressive, and if you have practical experience in a number of different languages and technology stacks then you are well on your way to grooming yourself for a future architect position.
I switched to .NET when my company switched sides because of client pressure to ONLY support Microsoft technology. This is the best way. If this option isn't available to you then try learning the language on your own and see if you can contribute to open source projects for experience. Contribution to open source projects is a plus on a resume.
As far as the SCJP, I took and passed that several years ago and I don't think it helped me get a job in anyway. Companies in the United States don't really look at programming certifications anymore. I am NOT saying you SHOULD'NT take the SCJP, because it actually helped me memorize important parts of the common API and really understand Java at its deepest levels. I DID learn somethings when studying for the test so I would go for it as a learning exercise only.