The great and awful thing about english, and other languages, is that there are so many ways to say the same thing. As a native english speaker i dont hear any difference between the two options but if i had to pick, i would pick choice B. Choice A is definitely more casual/colloquial, however you can make choice B less formal by combining "we" and "have" to make "we've". For example, "it has been a long time since we've..." this is te way that i would say it. Sorry i couldnt explain any better!