Java, J#, C#?

Hello sirs.,

First of all, sorry for my English, I'm brazilian.

I am a PHP programmer from about 6 years and last year I founded a company (Auriumsoft LTDA - www.auriumsoft.com.br) focused to develop Mobile App.

The fact is that I'm lost of what language and plataform to choose. I even thought in choosing C++, but I have a lot of hurry, so this is impossible.

What do I want to do?
- Develop enterprise mobile applications for Pocket PC, its desktop connection and transfer software, and web interface for generating reports, etc.
- Develop end-user app. for new generation Windows.
- AND NOT SO MUCH IMPORTANT: Have security if a client wants the mobile desktop software in LINUX, I could easy change to it.

The Plataforms
- JAVA does everything this without problems.
- .NET too, except the part of Linux. I know that exists MONO, but it isn't so compatible with .NET Framework (am I right?).

I want to have security, that the plataform and language I choose, I will develop stable, security and scalable enterprise mobile and desktop applications (I have experience developing PHP enterprise web based systems).

I know that both JAVA and .NET are capable of this.
But, I want security that if a client needs one of the desktop software on Linux, I would not say NOT, I would say YES. So, JAVA is the only one that can give me this.

So, I've thought to do this strategy:
- Study and dominate JAVA (J2SE, J2EE and J2ME);
- Study and dominate .NET using J#.

J# because I will know Java, so it will be easy, needing just to study the differences on .NET Framework, CLR compilant, etc. Am I right that knowing Java will be easy to go throught .NET with J#?

Or I just forget .NET and completely stay with JAVA?

Any help will be welcome.
Very thanks
Alfred R. Baudisch