asp.net is used especially for web designing while java is used for designing both thick client windows desktop applications as well as wed applications asp.net works together with C#, whers..java cant be much compared to java...but goes well with C#.C# and Java are programming languages...and ASP.NET uses C# for programming...
if i wud have to choose the better option,i think that using C#.NET(n creating web sites with ASP.NET) would be better,
i hope it helped you

Both the technoligies have their own advantages.

.Net
1) If the application is Windows platform specific then use .Net
2) .Net is Language independent, so if the team has multiple skill expertise C#, VB.NET , C++ , developers can still work on the same project with different skill set.
3) MS technologies provides RAD (rapid application development) to deviler project faster, customers always prefer faster delivery.
4) Debugging is very effort-less therefore, can fix the bugs quicker.
5) deployment is very easy and simple
6) Ajax implementation is simple & easy

.Java on the other hand
1) Platform independent
2) Open source
3) Dependant on the 3rd party tools to develop applications

Mohammed Yousuf uddin
Software developer