Exactly this is crucial test for both teams. Win would be huge boost and prove they are serious title contenders. You argue that Liverpool as defending champions have advantage because of their experience and consistent form. Your idea that since Arsenal beat Manchester United and you think Newcastle is better than Manchester United then Liverpool should be even better than Arsenal is creative way to look at it. This match is chance for Liverpool to show their dominance and for Arsenal to prove they can challenge best.
This is a big match for both teams in the Premier League, and both are eager to prove their best after marking the start of the season. A win will certainly be a motivator for either team, as it will be the beginning of a title challenge. The reasons could vary depending on which team will win the match, but I see it differently, as there is a possibility of a draw, reminding both teams that they will be cautious enough to avoid dropping points. The goal productivity of both teams is also balanced, only Liverpool is slightly better because it managed to score seven goals in the previous two matches, so that does not mean Arsenal should be underestimated because they also managed to score six goals in the previous two matches.