Inter Milan is surely team to watch in coming matches with their current performance is surely at the top, and they have enough quality to win against any team with in last few months mostly are talking about Manchester City, Real Madrid and Bayern Munich with these teams no one talking about Inter Milan as they are not near these teams but most chances they are having fair chance for the win and their game against Atlético Madrid is surely going to be tested case which will give all review about them if they are able to win then surely they could be in good position and also have good chance for the win.
Atlético Madrid is also not easy team but their performance i surely not near to beat Inter Milan with their home game can give them some advantage but still for the game which is going to play at Inter Milan home could be not easy for them.
Yes, Inter Milan is in form lately, but when it comes to their game against Atlético Madrid , I don't think they will find this game easy to win because, I have known Atlético Madrid as a stubborn team when it comes to anything pertaining the Champions League games, despite that they have not been consistent with their performance in a couple of games now but ATM got good experience to play and even deny any team the opportunity to win regardless of how form the opponent team is at the time of meeting.
Well I can that when it comes to the UEFA champions league there is totally different atmosphere and also a different vibe altogether and that's why even the team with the poorest form do well in the champions meet up. Just like you said Athletico Madrid is not a new name in the champions league and although inter Milan has the more advantage prior to their performance I will like also grade athletic Madrid a better chance at qualify for the knock stages of the champions league.