Not much difference - Go for J
There is not much difference really in the lounge - apart from the Hagen Daaz and Krug. J class has Champagne too... dont know which though.
The Bangalore flight is short, 4 hours, and you arrive at 10:30 pm local time - not much point sleeping. Onflight, apart from the seats the only difference is the Bulgari kit and I dont know what value you'd put on it.
So unless you plan to do much sleeping in the lounge, there is not much point in upgrading. Me, I'd use the extra $$s to upgrade to the Leela Palace hotel near the BLR airport and have me a great meal in their restaurants....