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.