The two arguments aren't exactly equivalent. Gun buyers are US citizens have a lot of rights and obligations from the government. Immigrants are not citizens and don't have any rights or obligations to fairness from the government. We already have very different rules for immigrants from different countries. A Canadian can get into the country much easier than other countries for example.

I don't personally support banning Muslims because the terrorism is so low it's not worth worrying about. But one could imagine if it got much higher and was a legitimate large threat. Then I would be completely ok with banning Muslims.