It depends what you consider as better...
for me, since I have some eye problems, I would choose the 1366x768 resolution because on the same screen the things would appear bigger than at 1600x900. (assuming both will scale to full screen size)
I would also tend to think that 1600x900 is more "standard" and will be supported by software more easily.
Hello. why do people ask questions like this? yesterday some db at staples was trying to convince me that 1920x1080 was "better" than 1920x1200. Yeah, if all I did was watch db movies, sure, db. He was blaming my choice of software instead. stupid salestard.
regardless of one's vision, higher resolution is -- you guessed it! -- higher resolution. we have 15" 1920x1200 laptops at work and they're fine to work on.
unf. in this day of walmartian excess, all the LCD screens have gone 16x9. as one dellite said the other day, "All our laptops are HD now, with high resolution!"
not the planet i picked, awaiting departure
P.S.: Appropriately, Kontera has picked up on the only two things that really matter, right?
Well, 1600x900 allows you to see more on the screen than 1366x768. However, everything would look smaller on a 15.6" screen. If you are like AntiZig who has less than perfect vision, then 1366x768 is better.
If you play games, then the higher resolution will mean lower performance if they are using the same video chip.
I would say go to a computer store and take look at the available laptops. Hopefully they will have 15.6" laptops with 1366x768 and 1600x900 resolution. You can then decide which one you prefer.