Here's a little clue, when you use the word "either" it's implicit that there's a choice. You aren't forced to join the Circle regardless of what you would like to believe. It's a choice.
in the game world yes it is a choice but I am talking about the person playing the game, they are not given a choice you can not do anything else in the companions questline until you complete the silverhand quest not even sidequests, from what I Have seen forces you to become a werewolf to get any further in the questline, the only other dialogue option you have when they try to make you a werewolf is I am not ready but that does not skip the quest now does it, there is no Dialogue option for I don't to be a werewolf like the I don't want to be a vampire option in the dawnguard questline
I hope I'm not spoiling you; but when you finish the questline You can cure lycantrophy by bringing a Glenmorilwitch head to the tomb and defeating your wolf spirit. If you're playing on PC you can take the easier option and do it per console commands, alternatively.
But as soon as you cure yourself you cannot enter the Underforge anymore, nor' can you get it back (unless you use console commands, not sure about that though).