I would take AI.
It’s non stop vs two short 7 and 6 hour flights… neither of which are long enough to get a good sleep.
Depending on which flight is chosen on SQ, OP may not need a "good sleep" anyway. If it's SQ 238->SQ406, you leave MEL at 9:15am (or 10:15am in DST) and arrive at DEL around 8pm. Entirely day time., and probably enough time to get to a hotel in Aerocity to get a good sleep and get back to the airport the next morning for a 6:20am (10:50am body clock time!) departure.
At the other end of the spectrum, if its SQ208->SQ402 then 6:25pm (7:25pm) departure and 5:40am arrival in DEL, then yes sleep would be a factor. But it's not going to make a 6:20am departure to Nagpur!
