How are the outlandish visitor at Arthur's feast and Sir Bercilak, Gawain's host at the castle, somewhat different?
They are very different. One thing they both have in common is that they are mysterious and there is a magical air about them. Other than that, they seem to be completely different. For example, the visitor to Arthur's feast is completely green. He is challenging and boastful. He allows Gawain to cut off his head and then, to Gawain's and all the other knights' surprise, the visitor gets up, tucks his head under his arm, and challenges Gawain to find him in a year.
Gawain's host at the castle is not as challenging, but he is up for a "game". He proposes that whatever the two of them get each of the three days that Gawain is his guest that they trade. Gawain gives him the kisses he gets from the host's wife, but he keeps the "magic" belt/sash. In return, the host gives Gawain the game he gets on his hunt. The host is more friendly and jovial than the visitor to Arthur's feast. He is a typical country gentleman with a wife and a posh life, willing to share with a guest.
We don't know it at the time, but the two are the same person. He purposefully hides his identity as Gawain's host to test the knight's integrity and noble nature. By keeping the sash, Gawain shows his human frailty and fear of death but he also lies. This is not a noble characteristic.
In the end, we realize that both the visitor and the host are attempting to improve the behavior of Arthur's knights. He is teaching them the moral lessons he feels they are lacking.