Updated. You can always wait for a new iPhone model to make sure to get the latest hardware. But never has the wait been more crucial, and it is not because of increased processing performance.
Do not buy an iPhone 5 now. There are two major reasons to go for the iPhone 6.
The first major advantage iPhone 6 has over iPhone 5 is size. Apple has lost huge market shares because the world wants cheaper and larger smartphones. They tried to deliver cheap with iPhone 5c, but failed. Now they try to deliver big, and certainly seem to succeed. If you don´t understand why you would want a bigger screen, you probably haven´t used your phone for what the smartphones is built for. Everything. A big screen will make your tablet and your computer obsolete (if you are a normal consumer). Wether to go for 4,7 or 5,5 inches depends on pocket size. 5,5 inch would almost rival Samsung Galaxy Note 3, but certainly beat it in sleak design. The same goes for the 4,7 inch screen vs Samsung S5.
Update 2: The following segment is irrelevant until (hopefully) iPhone 7 since Apple scrapped the sapphire glass.
Size is good, but the key selling point is most definately it´s sapphire glass. From the tests it seems to be pretty scratch proof and unbreakable. If it it lives up to it´s promises, the sapphire glass will be a revolution in the smartphone world making your mobile life less worrisome. And not to mention how much money will be saved from having to fix a broken screen, or worse, buying a new phone.
Update: Seems that the initial tests was not sufficient and that the Sapphire glass is not pure enough to withstand scratching from very hard material. Still a lot better than iPhone 5.
Apple has done a brave turn towards green and is following it up with iPhone 6. With the screen size of iPhone 6 you probably will find yourself not bothering to reach for the iPad for almost any tasks. Some people still will prefer to have an even bigger screen, but for most of us will be just fine with a phone combined with airplay on a TV screen. Apple is in away cannibalizing it´s own market, which is good for the environment.
Wether or not sapphire glass will trouble recyling, the increased life time of the iPhone 6 will certainly outweigh any problems. If it was not for the lousy number of charge cycles of Apples batteries, iPhone 6 can live for years and years. Although Apple has made it increasingly difficult to change batteries in their devices manually, they deserve credit for their battery replacement program. So why not make this your last smart phone this decade.
If only the worlds gadget producers could stop tempting us with only small updates and start replacing their hardware only when they have game changers like this one, we actually might have a chance to make a better world.