Here I agree with you that PSG should not be favourites in London... Arsenal have shown that they can beat any club in the Champions League and PSG in my opinion are no stronger than Madrid at the moment... I compared PSG's game when they played Aston Villa away and it seems to me that Arsenal will play even better than Villa... But I still find it hard to make a prediction who will win... Both teams play well in attack and I expect BTTS and a lot of corners in this game...
PSG won't be favorite in London, but PSG is better than Real Madrid.
Real Madrid can't win big margin even met against mediocre teams, while PSG able to humiliate the opponents. Arsenal do have a good history, but sometime Arsenal is not consistent during big match.
The first leg Arsenal will win I think.
The bookies are also not favoring PSG... because playing in the Emirate it will be the Gunners.

Strangely enough Arsenal when going to the top they are inconsistent, but we never know if in this semi-final maybe they will not miss losing... we have seen the proof of slaughtering Real Madrid which is quite large.
Do you support Arsenal? Same here.

Haven eliminated Real Madrid in the quarter finals stage of the UCL, Arsenal are widely considered favourites to win the champions league and that's why so many people do not think PSG may have any chance of survival at the Emirates, but on the other hand PSG are a formidable team and also have what it takes to eliminate arsenal at this stage but it all depends on the technical approach of the PSG side which I believe is key for the French side if they must get a positive result.