Node.js is not a language. The language is Javascript.
Both are perfectly good choices, the main difference is the asynchronous approach of everything nodejs related. This increases the performance and scalability, but it's harder to debug and uglier to write and read (Learn about the callback hell). You need to get the asynchronous concept very well if you want to have the advantages people usually credits nodejs with.
This being said, you can program asynchronously with PHP, but it's far less common.
So if you are confident with PHP, you need the results fast or don't want to experiment go with it. If you want to learn or experiment, go with Node.
If the limit is 10-20 simultaneous players the performance shouldn't be such a a problem.
About the design approach, I would leave the details for last. MVC is very common and you will probably use something like that to organize your code but the choice depends on lots of factors and you should probably think about this after having the use cases / stories of your application defined and limited.
