I visited New Digha with my friends few years back. New Digha is not that good as we heard. In West Bengal there are only 2 or 3 beach so I think it was that much famous, But personally I feel Mandarmani is much better than Digha. In Mandarmani we can have Water Sports and many other fun activities in comparison to Digha.
The connectivity facility is also good as government is promoting both these places as tourist spots. There are trains in the morning and also bus facilities at regular intervals and it is a journey of almost 3-4 hours from Kolkata.