This is very subjective. Some people can discern higher frame rates, while others can't.
Most people consider ~60 FPS to be preferable. This is especially true since most monitors can only display up to this.
For lag, most people say at least ~45 FPS for non-discernible lag.
For the 50-60 difference, I personally wouldn't notice in-game. Others might though.
Most people can definitely see more than 30 FPS. Though its true that eyes don't work in FPS, we can still tell the difference between frames at different rates. This varies among people, and what they are used to. For example, I've known people who were used to 60FPS for a while, then go to a 144Hz monitor at 144+ FPS and can't tell the difference. Then, if they go back to the 60 FPS after a while they would complain about horrible lag.
Another example is fighter-pilots. There have been studies done that show that they can identify an aircraft hidden in 1/200th of a frame (I think I got that right, it was at least 200 anyways. I can find the source if you wish.). This means that they would be able to discern 200 FPS.