Well, from what I've read, the difference lies in size, and that's about it. The plus is physically bigger, weights a few gram more, and has a bigger screen.
The other difference is in price, where you pay more for the bigger one, even though both smartphones are practically the same, except in size and weight.
My source is almost one year old, and then the price gap was about $100. Seems like a nice ripoff, or a sweet discount for people who prefer smaller phones.
iPhone 6: Curved aluminium, gold or light/dark silver, 6.8mm thick, 129g
iPhone 6 Plus: Curved aluminium, gold or light/dark silver 7.1mm thick, 172g
The iPhone 6 and iPhone 6 Plus look identical, but they do inevitably differ in the size and dimension departments. With the iPhone 6 Plus, you’re getting a 2cm taller, 1cm wider and 2mm thicker phone. It doesn’t sound like a lot, but it clearly makes a huge difference when you handling the two iPhones.