There is nothing much sceneric on both the routes... I would always prefer the Bangalore to Munnar route, because its shorter compared to Chennai route and its not hot like the Chennai route... Only after reaching Foothills, you'll find it cool, until then it'll be way too hot...Beat the heat by drinking The Famous Tender Coconuts of Pollachi...
Happy Travelling!
Chennai to Munnar is more scenic. Best route is to take the GST road NH 38, it will take around 10 to 11 hours, and you have to cross 7–8 toll gate, if u go in car it will take around 10 hours, if you go in bus it will take around 12 hour.