Real Sociedad are a much better team in the Champions League compared to the La Liga this season. They completed Group D as leader despite having a competitive opponent like Inter in the same group. Can they be a challenging opponent for PSG as well? I think that they absolutely can. You can predict the reason behind that easily too in fact. 
Real Soceidad performance in the La Liga this season doesn’t speak well for them to be able to go far in the Champions League this season. After their qualification from the group stage, and they were drawn against PSG, I just imagined that they will not go further in this competition and this will be the end of their journey in champions league this season. How I wish their performance in the league game is a a good as that of PSG, I would have given them some hope of winning the match and qualify for the quarterfinals.
I'm on the board for City not winning back 2 back UCL as this is very hard to achieve and no matter how strong your team is , UCL is a very special competition where personally I saw so many surprises in watching this competition for over 20 years. However, with a bit of luck and Pep's brilliant ideas , City can produce a big surprise and be the second team who goes back 2 back UCL after Real.
It will not still be easy for Manchester City to pull this through for the second term consecutively. They look so good and confident of getting the victory this year again but it won’t be an easy ride for them. Real Madrid did back to back not long ago but for any team to do that now, it is not going to be an easy task for them. If this eventually happens, Pep Guardiola will be the best coach I have seen coaching a team in recent time having achieving perfect results from Manchester City.