Why do you think Federer has more chance to win the Olympics? Grass is his favorite underground and he already won 8 times Wimbledon, so I think if Federer wins another title it will be Wimbledon.
it is number of matches and "best of five" sets rule, that I think he is not able to pull out seven matches in two weeks, no matter the surface being as fast as grass, his favorite
on the Olympics, it is supposed only final to be on "best of five", so it is easier to get those matches in short interval, and he is good enough on hard surface as well, so there is his chance
nevertheless, I think that his chances are slim on both tournaments, he is just not up to his game, due to ages and injuries, and other players are in good shape, too
with all that said, can't say that I would not like to see him getting another GS
I'm not to sure that hard would suit Federer after coming back from his knee injury as hard courts can be hard on the lower part of the body especially the knees. Also if the courts are fast then I don't think it will suit him which is why I think Wimbledon is a better fit for him right now as its less strain on the lower part of the body. Anyway lets just see what happens because he first needs to get 100% fit and healthy.
To be fair, any surface would be difficult for Federer now, assuming his body is really ageing, evident of his injury now that took a long time to heal. But him being a competitor, he will push his limits again if he ever comeback and it might hit him hard that could end his career or think of retirement.
In other tennis news: https://www.firstpost.com/sports/atp-tour-hands-sam-querrey-probation-suspended-fine-for-russian-covid-19-flight-9161391.html