From the links below you will find the required information. You can even compare the different processors that you want.
The only difference I can find here is that the T6400 has 2MB cache and it is a Core 2 Duo Processor.
Also with the new Intel processor ratings, the T4200 has only a 2 stars ratings whereas the T6400 has a 3 stars ratings, making it better. But this depends on the usage.Link: http://www.intel.com/consumer/rating.htm
On the overall, personally I would recommend the T6400 as it is a Core 2 duo, it will definitely perform better than the T4200 which is only a Pentium processor.
Hope this will be helpful.