Injuries and how well teams play will definitely affect their chances of making it to semifinals making competition even harder. Liverpool and PSG are both trying to win titles but they are in different situations. PSG can focus only on this competition because they are already ahead in their own league Ligue 1. On the other hand Liverpool has to deal with very tough competition in Premier League which might make it harder for them to focus on both competitions. If they make one mistake they might lose their chance and game will become even more difficult. Result of first game will be very important and PSG might have small advantage because they get to play at home.
To see the quality of both of them, maybe Liverpool is a little better because they have experienced much tighter competition in the Premier League so they are used to facing big pressure in the Premier League, they will be much more used to facing pressure in the Champions League. In contrast to PSG who only face much easier competition in Ligue 1 so they are not so used to facing big pressure although that is not the only reason that we can use as a reference.
The result of the first leg will be quite decisive because whoever wins the match will have a much better chance of advancing to the next round and perhaps this opportunity will be utilized by PSG when they face Liverpool later.