The one difference that honestly creates a substantial difference between the two is the screen size. Which size do you prefer? OIS is a negligible difference. The higher resolution will deliver a bit more detail (around 70 more ppi if I'm not mistaken). The biggest bonus is the increased battery life, but that's naturally because of its massive size. Why would it run terrible? The difference performance-wise is hardly there unless we're talking about performance in games. Right now the stuttering/lag happens on both devices thanks iOS 8 being god awful. Take a look at Anandtechs benchmarks and you'll realise that the only thing it might bottleneck in might be 2-3 years ahead in really graphic intensive games, but I even doubt that since developers usually try to find a sweet spot to sell for as many devices as possible. I'm well aware of the fact that the 6 Plus goes through a downsample, but if Apple really pull their **** together, then their future updates should improve performance for both devices. Remember that any badly coded software can ruin or slow down state of the art hardware.