There has been no announced date for the release of Lightroom 6. It is expected by many that it will be released sometime during 2015. The rumor mills are saying that it will be released in March, but those are only rumors. Anyone who really knows has signed a nondisclosure agreement which restricts them from providing that information.
Something to consider would be to sign up for the creative cloud version. You yet Lightroom and Photoshop for $9.99/month, and when a new release comes you get it automatically as part of your subscription. If that doesn't appeal to you then you will have to decide whether or not to wait. Those who purchase one version of Lightroom within 30 days of the release of a new version have historically been able to get the new version without additional charge. But since no one really knows when the release date might be, your guess would be as good as anyone else's.
Lightroom 6 will probably have some new features. Since I don't work for Adobe, I have no idea what those features might be. I would expect that we would get the brush in the gradient tool and the brush tool that enables you to brush away part of the effect. But that is only a guess. Once Lightroom 6 is released there will be no more updates whatsoever to Lightroom 5. That means that if you purchase a new camera that is released after Lightroom 5 support ends you will not be able to work on the raw files from that camera. Personally, I think it's always a good idea to keep current with Lightroom. It just keeps getting better. Personally, I think the creative cloud package is a very good idea. I use it and I'm very happy with it.
But do you think it will be worth it to wait, or is Lr 5 and Lr 6 not going to be that much different?
Nobody who knows can say.
There will always be major new features in a totally new version of Lightroom.
Whether or not you use those features is a different story. And since we don't know what those new features will be, we don't know if you will use those new features to make it worthwhile.
So make a guess, and either buy LR 5 now, or wait for LR 6. That may seem like pointless advice, but it really does boil down to total guesswork right now.
Unless you are really strapped for cash, I recommend buying Lr5 now, and Lr6 when it comes out, and don't think twice if you just missed a cutoff..
That way, you are running the best version of Lightroom now, and will be again in the future, and your money is helping to support the software you use.
The moral of the story: don't be penny-wise and pound-foolish by trying to give a minimal amount to the company which feeds you.. - send the message: "I want the best version of Lightroom you can produce, and I'm willing to pay for it - thank you..".
That said, Lr6 is likely to come out fairly soon, presumably no more than a few months..
PS - There is "always" something enticing for everybody in a new version, despite the features you couldn't care less about.. Lr6 will probably have face recognition (which you may or may not care about), but there will no doubt be some improvements to the develop module too etc..
Rumors have March 9 as the likely release date - The box image, new features and specs have already been leaked out.
There are two things you can do if you want to use LR5 now and not pay to buy both versions: (1) Use the 30-day free trial or (2) buy LR5 from Adobe directly and take advantage of their 30 day return policy before the 30 days is up.
With most product releases they also have a free upgrade policy to the newer version if you buy the outgoing version right before it's released (here's more info). It's actually a great time to buy Lightroom 5 as most resellers have good deals on LR5 now so you can buy LR5 at the sale price and get LR6 for free... The only risk is if the release date is pushed out by Adobe and you fall outside of the free upgrade period.