When learning a language, you have to think in that language. ~니다 is the most formal out of the 2. ~ 요 is polite formal and what is most commonly used among koreans
어서 ~ an old way to say hurry up. 오다 ~ to come ~세요 polite ending to ask somone to do something 어서오세요 - please come in quickly. Mostly used among people working in a store.
환영합니다 is welcome to..my country, my city, my home, etc.