The best pc for software development is a desktop, not a laptop.
With a desktop, you can have multiple large displays which is very helpful.
A desktop will give you more performance per dollar.
On a laptop, you are restricted to smaller 17-19" screens and only one.
Just how much do you value the portability capability of a laptop?
portability is a must.... for automation applications startups, you need to go to the job site. Also a native serial port is a must have but no laptops have them now.
I'v had very little luck with usb to serial converters. Often I need to connect to older controllers and othe devises and I had to make up some crazy connecting cables. The converters are fine for a mouse of a modem but when you need to wire all the pins, maybe at both ends, sometimes with jumpers and such across some of the pins or send 5 vdc out the convertens nerver seem to work.
Also, screen depth is a must to see as much code or ladder logic as posible. Todays laptops are for games and movies but I don't get paid at a startup for that.
Just my two cents...