Personally, I prefer the extra space a 17" machine has. The picture quality is the same as a 15" computer but the extra space in the width is very useful in Sketchup and Layout because of the “panel” interface. If you leave the various ‘trays’ on screen while you draft.
The processor should be a “quad core”, the machine should have a ‘discete’ video card, not built into the chipset and the ports, (USB etc.) should be near the back, not the front of the computer. Extra cooling fans are good, touchscreens are bad and Solid State Drives are the bomb. Like Electric Cars; Once you try one you’ll never go back.
I’ve gone from notebook to desktop, back to notebook and now I’m thinking about an “all in one” machine. The setup of just a large, curved screen, a wireless keyboard and mouse and my Spacenavigator seems perfect. I’m also dying to try out a virtual reality headset as a monitor soon. If that works, it will be the ultimate setup for a while.