This question has been posed before and it's pretty simple actually. They both have pretty much identical powers, to control matter and energy, convert one to the other and vice-versa. They've both created new universes and they've both had their malevolent sides split off as separate entities. They both even got their powers in nuclear reactors. So, assuming from the similarities they have the same power levels, it would come down to a battle of wills. Phil Seleski (Solar) is a well adjusted guy with a healthy self esteem. Pre-retcon Owen Reece was a callow individual with almost no self esteem whose psyche was so weak that he unconsciously limited his own powers. In that battle, Solar would win easily. When pitting modern versions of both against each other, it would be a draw.