Hi,
That shouldn't be the approach to start with.
You should base your software stack based on your client's existing technology stack.
Otherwise, the client get into the trouble of maintaining two difference stacks.
IMHO, both .NET and Java were well matured now and so it wont be wise compare it at basic level.
So it is better if you can try to address what exactly Java web services can provide solution for a specific problem that .NET can't.
Regards
Balaji