My 2005 Acer 15" laptop weighs 3kg and is thicker.
The MBP 17" is slimmer and weighs only 2.99kg (a record for a 17", even nowadays), and still has an impressive battery life.
My point is...it's the slimmest and lightest 17" laptop in the world.....its measures and weight were considered normal on a 15" just 4-5 years ago....so, IF you need the real estate of a 17" screen, don't think about it twice....you're getting the most portable 17" on the market, maybe more portable than some 15".....
If you don't need that screen, go 15". (+ refurbished Apple Cinema Display LED at home)