They are both powerful. But the winner would always be the same ...Hal Jordan.
His willpower is just too powerful,and the GL ring operates on will power.
Even if you were to go to their uber-forms ....with Hal being Parallax and Kyle being Ion (basically both of them having all the powers of the GL energy), Hal would still win. Even though they are equally powerful Hal has more will power.
The rings and energies of Oa use will power.
Hal wins.
Everytime.
(Unless he decides not to win there is no stopping him).
Hey, in the Parallax arc he decided not to win. The guy had already destroyed the universe and remade it in his own image.
A similar situation is Marvel's Thanos.
Everytime he wins (eg like when he had the Infinity Gauntlet), but at the end he decides, always, to lose (subconscioulsy) and is thus defeated.
Thus winning ....until deciding subconscioulsy that it is time to lose.