While discussing the top contenders for this season's Premier League, I agree with you that Arsenal and Tottenham are among the top contenders, but we must also consider how Manchester United is attempting to showcase their own team work; Manchester United is the only team to have defeated Arsenal, and they have only lost their first two opening matches; I believe they are now strong enough to be title contenders as well, if their recent form continues.
It's still early I think but indeed for now the toughest are them (Arsenal and Tottenham) but in this case, I think there will be more as time goes by some other big teams will definitely follow because their performance is starting to change like MU or Liverpool although for Liverpool at the moment it still looks creeping but I think when there are improvements both internally and in the scheme, they will definitely come back to the top.
When it comes to the title race I think Manchester City is still a strong candidate, even though Arsenal are still top of the table but it's still not strong enough for Arsenal to keep it. Also, so far Arsenal have not played against other big teams like the big teams in the English Premier League, I'm sure when Arsenal play against rival teams like Tottenham and Manchester City the results will not be perfect. But if Arsenal can beat or at least be unbeaten against rival teams then obviously Arsenal have grown to be stronger but, against Manchester United alone Arsenal can be beaten by a score of 3-1.