I remember in January 2013 getting the 5 (over a 4). When the 5s came out, I regretted getting the 5, as the 5s was much better - had a more solid case, no weird recall issues such as the battery/power button, and had Touch ID. Ironically (and fortunately), my 5 was stolen in February 2014, and AT&T was kind enough to allow an early upgrade to the 5s, so I grabbed it. Haven't looked back since then. (And, in a case of good karma, the 5 was recovered. Sold it unlocked, made a neat tiny profit.)
It depends on what phone you have right now. If you had the 4s or older, I might go with the 6 now, depending on your tolerance level for a slow iOS 8 experience (or iOS 7). If you have the 5/5c/5s, I'd just wait until the 6s/6s+ or whatever it'll be called. It's also worth remembering that availability of a new iPhone is usually scant until around late October/early November.
Based on past patterns, the "s" release of the iPhone is usually better than the original release. The "s" release builds on and improves upon the original version, while the original releases are more revolutionary and bring major changes. Kind of like an OS X Leopard then OS X Snow Leopard pattern, or a Lion / Mountain Lion pattern, if you may.
Hope this helps!