My Solar Industries panels are 2". Their standard panels are 1 1/2". I wanted 2" because my pool was plumbed that way and we have a long way to go to the panels (130' from the pump).

A very reputable installer I spoke with initially wanted to put in 1 1/2" ("we do all of them that way"). When I insisted on 2", he called Solar Industries and they recommended 2" for our installation. To his credit, the installer was very gracious and basically said that you learn something new every day.

Cost was the same. I think the real difference only comes if you had enough panels to "use up" the 44gpm of 1 1/2" pipe. From what I remember, most panels can handle 4 to 10 gpm per panel (4' wide - one way, not u-turn type). So that means that if you are planning on 4 or less panels, 1 1/2" is fine, between 4 and 8 or so it is a toss up, 8 or more will really benefit from 2" pipe. (I have 8 4 x 14 panels). All of this depends on how much flow you push through the panels. The more flow (without stressing the panels), the better (see other threads on solar efficiency).