Every model brings upgrades OP, the camera is one area iPhones are consistently excellent in, but the S models have brought some big(ger) camera features
- 3gs was a big improvement in 2009 over the 3g/original iPhone, and brought video recording
- 4s brought probably the largest camera leap from generation to generation...an incredible camera update all around
- 5s brought improved low-level-light pics so much that it still holds up to the 6 and other recent flagship android phones today
6s is reported to bring a big leap, but it remains to be seen.
Personally, I think the 6 camera is "good enough" and will hold up with flagship phones for years. But it depends, do you want the latest and best camera for another year? Or are you content with getting a 6 now and using that camera for the next year
I also think that since we are so close to the next iPhone reveal, it would not be worth getting the 6 just yet, if you're signing a 2 year contract. Wait to see if the 6s suits your needs in another month - and if not, then you can get a 6 on contract then, and it will be cheaper than right now (unless you can find a good deal right now).
The 4S camera is still very, very solid to this day so I'm sure the 6 camera will still be great years from now.
edit - OP check out this link, its a nice comparison of every iPhone camera
http://snapsnapsnap.photos/how-does-the-iphone-6-camera-compare-to-previous-iphone-cameras/