There are objective criteria where one can be said to be better than another.
I think the 6+ is the more polished product, and if you have a Mac then it almost isn't worth considering the Note 4 over the iPhone 6 Plus.
I plan on getting a MacBook Pro next month, but still giving myself some time to consider the Surface Pro 3 and wait to see if Google announces some changes to their services before I change my daily driver. If I get the Surface Pro 3, I'll keep my Note 3 or maybe consider upgrading to the 4. If I get the MacBook, I'm getting the iPhone 6 Plus - it's a no brainer with the Continuity and Handoff Features, as well as FaceTime and iMessage which works on both devices, flawlessly and without 3rd party apps or plug-ins.
Having an M8 and a Note 3, I don't think the actual device matters as much as the ecosystem backing the device, and how well it integrates with the PC Operating system - that's where most of my productivity gains are to be had (which is why I tend to use Microsoft Services on my Note 3 - I can start editing in Office on my PC and when I open it on my Note, it goes to the same place where I left off on my PC, etc.).
Both devices (M8, Note 3) are practically on par with each other and I don't see the Note as being more productive than any other device because the form factor is terrible for Note Taking and very very few 3rd party apps aren't optimized for the S Pen or Data Sharing APIs - which relegates the Note to nothing more than a slightly larger LG G3 with a stylus to poke around and draw in certain apps, in the grand scheme of things.
I really think that I need an Actual Tablet if I'm going to take Stylus Notes on the Go (which is why I'm considering the Surface Pro). That, or just give up on that idea and use a decent laptop with a good developer ecosystem and an OS that integrates flawlessly with my smartphone.
You can get Wacom Tablets for a Windows or Mac Laptop for < $100, anyways...