leave about speed ...punctuality wise both are on par..however if you wana have a well maintained train interms of cleanliness and facilities like charging sockets n all then prefer TG...if you to have a convenient departure @NDLS then go for TN...i traveled in June ..booked in sleeper from WL-NDLS for this TN and couldn't continue my journey due to pathetic cleanliness(coaches are not clean and even berths are quite muddy with dust, no charging sockets) and conditional of coaches...paid the difference amount to TT and shifted to 2AC which is rather well maintained...thats my personal experience which is quite recent..
so i will prefer TG over TN any day from here on :)