Display – This is obviously personal preference, but I find the 6+ display much more comfortable. Truth be told I never found the 1520’s size quite comfortable, but it was my only option as a 5" screen was just too small for me.
RAM – Yes the 1520 has more RAM, but I’ve seen no use case yet where the 6/6+’s 1GB of RAM has hindered user experience (I’ve even had mutliple pages open in Safari in the background and regular tasks were performed fine). My 1520 on the other hand occasionally gets the "Resuming" issue, and at times has felt slow switching between open applications. Xbox Music is my biggest offender when it comes to this.
Expandable Storage – This is indeed nice to have, and ends up being cheaper in the long run than purchasing a 128GB iPhone, but since there is the option for 128GB it’s entirely personal preference.
NFC – Pretty useless for me here in Canada.
Battery – My battery life went down significantly with 8.1/Cyan, and I’ve been through three 1520’s (screen tap/sensitivity issues, then changed from a 1520.1 to 1520.3 for better band support). The 6+’s battery seems fine from user reviews, always an area where YMMV though.
File Manager – Never used it on my 1520. Not once. Niche feature is niche.
I’ll tell you want an iPhone CAN do. It can download all these amazing apps that have 3rd party alternatives on my 1520. Hell even Microsoft’s apps are actually better on iOS (Smartglass is buttery smooth, MSN news looks beautiful, the RDP client is fantastic!). I’ve been using Windows Phone for 2.5 years (Lumia 900, 920, 1520) and I’m probably going to switch because of these things.