Depends on your interest and what have you done before pursuing M.Tech. Personally if you have B.tech in DT then go for DE (it will increase your knowledge on process engineering). If you have done any other engineering/Science then go for DE,it will broaden your perspective and chances to many other food industries. If you want to be a product specialist/Expert then go for M.Tech DT.

In other words if you have interest in engineering than opt for Dairy engineering but if you like kinda technology things like processes and chemistry behind them go for Dairy technology.

Generally people opt for Dairy technology because most of the recruiters ask dairy technologist only. But in last i just want to say follow your interest.

I hope this will help you