If you want to play now, and not wait, I wouldn't wait until 6th edition. If you have 2nd edition books, you could still play that edition of course.
If you decide to get the 5th edition, I'd just get Core for now. There's plenty in it, and more stuff just makes the system that much harder to learn. Maybe have a look at 6th when it comes out, so you have a basis for comparison.
If you do want all the extra books, Run & Gun, Street Grimoire and all the others, you should definitely go with 5th, because it will be years before 6th has caught up with 5th on that front.
On the other hand, if you want to start small and are willing to wait, you might as well wait for 6th.
I'm sticking with 5th for now. I just started a couple of months ago and have sunk a lot of money in 5th, so I'm not eager to drop that investment. Maybe I'll have a look at 6th when it's a year or two old.