I really doubt on a somewhat basic pool that the cost difference between 1.5 and 2 inch PVC is more than $50-$100. Other than that small out of pocket there is NO down side to using 2 inch. 2 inch will be better, will allow to not work as hard / do more work on the same amount of power, how much more depends on how close to the edge it would be with the current flow requirements, as well as how much restriction there is in the plumbing layout. Think of this like having an option on a new car that would promise to give you better fuel economy and perhaps better overall performance, but when asked to quantify it the best answer we can give without detailed study of your pool plans is between .5 and 4 mpg improvement and maybe more responsive performance depending on how you drive. So even worst case and it is a .5 mpg improvement, in the long run eventually that $100 initial cost will be paid back on power saved.
Ike
p.s. one thing to realize about pool builders, most do not look at the long term view, they layout pads to be compact and "neat" with no thought to leaving space to splice in replacement equipment, or include extra union fittings to make such jobs trivial. They install 1 speed pumps that are cheaper even when a 2 speed would pay for itself in power saved in a year or two (perhaps less), etc.