I do all of my painting on a 20" iMac, but I think I want to change over to a smaller but more portable 15-17" Macbook. I guess my question is, will I really miss the couple inches I lose on the screen, and is it better to have a solid home base like that. Or do a lot of you guys use portable computers as well, I want to know what would be the more professional choice. I just figured it works better with my lifestyle since I can't always be at home. To me I can't really see a major difference, but decided to ask since it is an expensive decision.
15-17 is a lot smaller than 20, remember a screen that has 2x the diagonal has 4x the screen area.
An imac is basically a laptop that is stuck on your desk, so you will not loose much in terms of performance (both computational and display) by going to the laptop. If you were debating going from a desktop to a laptop, then you may notice some discrepancies performance wise, but in your case you will only notice a smaller screen. Also, the imac screen is pretty bad, definitely not "professional". With either machine, I would get a more accurate display. I know several people who have a macbook and a display and are quite happy with the solution.