Should I use Java or .Net for client project?

sir, i have a small Developing firm and used to develop using VB6 but problem is that my client now wants to get remotely access their business trend using both laptop and mobile and wants to get transaction message as well, moreover sir VB6 programs don't run in win8 op.System, i am very much strong in coding and logic please suggest me which platform i choose whether Java or .net. moreover in future if i want to do job in company which will be more valuable.