Hi Oswaldo,
I use my small router plane more often than my shoulder plane. But I have both because they do different jobs in my shop. For me, the shoulder plane is used on tenon shoulders. But like many tools, you can use it for other functions such as cutting camphers.
I bought the router plane first and I have never regretted that. I originally purchased it to make sure that any long dados I cut on a machine are cut to the exact same depth over their entire length. (Sometimes, a long part rises up just a little as it goes over the dado head. Then the joint doesn't fit properly because the depth isn't right. So I use the router plane to make the dado bottom uniform depth.) After I bought this tool, I discovered that it has many other uses that others described above.
I would give up my shoulder plane before I would give up my router plane. But that's just me. Everyone works wood a little differently.
Good luck!
Fred
"All that is necessary for the triumph of evil is that good men do nothing."
“If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals.”