See, not sure what to do, I keep going back and forth between the two.
If I got the Plus, it would render my iPad 3 pretty much obsolete. I've only been using the iPad recently because I don't have a phone right now, and it's easier to grab that than go on my Mac.
And after having a N6, I know what it's like to have a 'phablet' in my pocket so that doesn't particularly bother me, just I'm going to have a decent case on it which will add bulk.
I like the fact the Plus as OIS, but I keep thinking if I buy the Plus, when whatever version of iOS comes out next, depending what it does, is it going to run terrible on the Plus, when the 6 handles it great because of the lower res screen.
Decisions...
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.