I was intrigued by your question. I'm not going to explain the in-depth advantages and disadvantages to each move as you probably already know but just can't decide. In my opinion, Rock Polish is better. Groudon has a very average Speed stat for an Uber, so even if it gets in a Swords Dance, any faster pokemon such as Mewtwo can come in and revenge kill it with powerful special attacks. A Rock Polish allows it to outspeed these threats and still hit hard (it does have base 150 attack after all!)
However, if you are finding that it still does not have the power to muscle through extremely bulky walls like Giratina and Lugia, then a Double Dance set can be run, with EQ and Stone Edge as the attack moves. Swords Dance against slower teams, and Rock Polish against more offensive teams. Smogon even suggest an interesting set with SD and Thunder Wave, which also supports the team very well.
Overall I am going to say, if you are only running one boosting move, then choose Rock Polish - but they are both viable.