The "lag" issues are dramatically overblown on the Note 4. I've seen a lot of back and forth arguments about lag on previous Galaxy devices, but this particular little pissing contest is pretty overblown.
There are some places here and there where the phone might display some lag. Factors that seem to matter are the carrier (more Verizon users seem to notice it) and the process you're using (everyone seems convinced the recent apps button is a big one).
In my personal experience with the phone, there is virtually no lag at all. I have no lag in any of the areas commonly complained about (most especially the recent apps button) but I do notice it in some TW/Samsung apps from time to time. Its really not a big deal and doesn't affect the overall performance and function of this outstanding phone. In fact, there are complaints about lag on the i6 series once you get into the multi-tasking aspects of the iPhone. As much as Apple would like to claim otherwise, the iOS platform was never designed with multi-tasking in mind.
If you are a diehard iOS fan, the I6+ is going to treat you well. I'm not bashing iOS, I'm simply acknowledging that iOS 8 is a gradual step up over iOS 7, mostly cosmetic, with a few minor upgrades here and there. But if you like that ecosystem, it works for you and you have no complaints about it--go for it and don't think twice about it.
But if you like the idea of customizing your phone more, having a phone that has much greater multi-tasking capability, a much better display, removable battery, external storage, etc. then the Note 4 absolutely will not disappoint you.
My wife is a diehard iOS fan and wants nothing to do with Android phones. If her company ever decides to upgrade from the i4s series they've been flogging like mules, she wants an i6. My Note 4 is the first Android device I brought home that she said "I would almost be willing to use something like that" because of the S-Pen. She didn't particularly like the size, but everything else I showed her she really liked.
Both ecosystems have their place, and both do a good job for the segment of the market they serve. You just have to decide which market you are happier with.
Lag is not an issue I would concern myself with for making the decision, however. While there are certainly some issues with it, they aren't disruptive to the operation of the phone, and they are certainly no worse than anything being reported on the iOS platform, either.