ok please dont shoot me but im so terrible at waiting i would GRAB the black birkin if i were you. if you have sufficient means to get both, and you definitely do not have to miss out on your HG if you get either a birkin or lindy, then go for it. between the lindy and birkin, i would actually get the birkin, as you mentioned that this is a preloved one, so even if you decide one day that you dont use it that much, you can always sell it off and you wont lose as much. also a black birkin is such a classic versatile piece!! u can use it on days when you are feeling lazy or to places where you know there is potential for stains on your bags like perhaps putting it on the grass on picnics?? ok not sure if i am putting this across well, I have both a gris t and a black, and i use the black way more than the gris t. i noticed a black scuff mark on the corner of my gris t after the second time i used it, whereas ive used the black dozens of times and nothing at all! or maybe its there but i just cant see it cos its black.
ok so in short.. i would get it! BUT only if you dont have to give up your HG if it does arrive in a months time if you get it, KWIM?