The Question asked by you is a very stupid question.But since u have asked it now, here is the answerSL 399: Data Usage upto a maximum of 400 MB/month, with additional MB's charged at 1.20/MB. No additional feature, Monthly rental : Rs.399DSL 590NU : Data usage upto a maximum of 500 MB/month from 8am to 0000hrs, additional MB's charged at 1.20/MB. Monthly Rental: Rs.590Additional Feature of 590NU: Unlimited usage from 0000hrs to 0800 hrs, no limit on downloads in this period.Hope that was simple enough for you to understand.
Restart is not required.Even if u r online from 11pm till 9am mtnl will only charge u from 11pm to 0000hrs and 8am to 9am bec in NU 0000hrs to 0800 hrs is free usage. Its automatic billing u dont have to worry about restarting the router.