As long as it's in good shape..
Unless the 57 is damaged in some way, it's no coincidence that everyone is telling you to get that one. If you've only heard the comparison on acoustic, you've heard the 57 at it's worst. Of the 2, most would pick the 8000 for acoustic guitar, and possibly for drums if that's the only mic you're going to have available to record them with (i.e., sticking one mic out in the room to pick up eveything), but even that would be debated. For vocals, it's the 57 hands down. Same for electric guitar (but if you have more than one mic for that purpose, the 8000 makes a not-too-bad distant mic for picking up the sound of the room).
Get the 57 now. You can always add an 8000 later. If you get the 8000 now and add the 57 later, you'll have to pay more for it then. The 8000 will probably still be the same price later.
And don't forget that the 8000 requires phantom power, in case you don't currently have a board/pre that supplies it.
While you will hopefully add more mics to your collection as time goes on, you will NEVER outgrow your need for a 57, and you'll never regret having it. And unless you try recording underwater with it, 57's last forever.
My long-winded way of saying GET THE 57!
Good luck and HAVE FUN!!
