My understanding is second stage is harder, first because of the rocket equation. Every Kg on the second stage is much more costly than an additional Kg on the first stage.

Second, the second stage needs to go into orbit before it can return. That's much more speed and a harder reentry.

Third, you have to deal with thrust. Falcon 9 has 9 engines, which means the landing burn can uses 1 engine for 1/9 of the thrust. This is still too much power and F9 currently needs to do a suicide burn. You can't do that with only one engine on a stage that is 1/10 lighter than the first one.

Fourth, second stage has a vacuum engine with a nozzle not fit for atmospheric use. It's so thin and flexible it would probably crumble or disintegrate upon entry.