More planets = bettermy complain is it is very hard to tell how much potential a solar system has...
just how do i figure out how valuable my enemies system are and which to invade 1st?
Bigger planets = better
The individual planet yields aren't as important as the size and number of planets. There may be some exceptionally rare cases where you have several large planets and none of them produce any food without farms but it's so unlikely that simply going for more or bigger planets will be a sound rule to follow.