No one knows.
The usual Adobe policy in the past is if you purchase a version of Lightroom and within 30 days a newer version comes out, they will give you a free upgrade. The problem is, Adobe never pre-announces when the new version will be coming out. (And of course, Adobe could announce a different policy for Lightroom 6)
So, its up to you whether you buy now or wait.
Another point to think about - if you use the Creative Cloud version of Lightroom, you will get Lr 6 at no extra cost.
Thanks for this alternative, I'll think about it.