What are the upsides and downsides of choosing Rails over PHP? Is one language better than the other?

Not really.
Just that the difference is between the scripting. Ruby is different in the scripting style than PHP .

If you are at ease with PHP, choose PHP, if you are at ease with Ruby, choose Ruby.
You may want also to combine them. Because Ruby has its own functions, some better than PHP and PHP has its own too;

They are able to accomplish different task.

It's your choice !

Rails is a framework built on the Ruby programming language. PHP is just a language. You are better off comparing Rails to some of the PHP frameworks out there.