1) How much time do you have to practice before the event?
The more time you have, the more you can not only weigh your options but
learn and test them. After all, you don't have to finalize your decision until you register, so if you've got both decks and know how to play both decks, you can adjust to what seems best the day of the event.
2) How much can you afford?
If you've got the time to practice - I show there is still over a month before the North American International Championship - then you don't need to limit yourself to just these two decks. It never hurts to have a third option
unless it detracts from resources (including practice time) needed for the previous two. Even if the third option is a budget deck longshot if the meta looks right going into the event... which brings us to our next point.
3) What does the metagame look like?
If you can adjust to the anticipated metagame, you can get a very big boost indeed. If everyone is going in expecting wall-to-wall Buzzwole-GX decks, running something that counters what counters Buzzwole-GX can be worth it even if you're not as experienced with the deck as you are your primary choice. This should really
replace "power" if you're a nitpicker like me (if you're not a nitpicker, you're probably rolling your eyes at me even saying this XD). We're concerned with the most favorable match-ups, plus general reliability. Plenty of Pokémon have raw power, even decks have raw power, but aren't competitive because they are unreliable, have too many terrible match-ups, etc.
*waves at Charizard-GX*
*Charizard flips off Otaku*
Erm... oh, and this all can mean running something you're less familiar (but
not unfamiliar) with, that is one of the competitive (but not top) decks in the format can actually be the correct call. However, there is something that can easily trump that and it is...
4) What am I most comfortable running?
This ties into both being knowledgeable, and recognizing any quirks you may have with the game. "Playstyle" is a thing, the kind of deck, cards, even general plays you naturally gravitate towards,
especially if you're good at them.
The best players go beyond their personal playstyle, but that doesn't mean one should ignore it.