I find the 6+ to be great for movie watching. Of course a larger screen is nice. But, it displays very well and audio is good enough not to "need" headphones. That may vary by movie though. I ripped mine and keep them on the iPhone. About 6 at a time.
I bought the iPhone 6 Plus because of my Movies/TV Show library. The iPhone 5S was great, but the 4" screen plus the screen not HD 720p or even 1080p was disappointing. The 5.5" 1080p screen is absolutely amazing. Watching Full HD videos is crisp and enjoyable. Some of the Movies are letterbox like a typical movie on TV, while most of my TV shows are full screen 5.5".
I've warched quite a few movies streamed from xbmc to infuse on my plus. I tend to watch them in bed while my wife is asleep so as not to wake her. I find it quite enjoyable and the size is just fine.
Infuse also lets you store the files locally without having tonsubc with iTunes, so it's rather awesome in that way. I did find a bug that I was told to contact them via email about though. Haven't heard back, been abot a week, but holidays and all...
I usually don't watch movies on my Plus because I'm generally near larger TVs that fill the need better. However I do watch YouTube videos from time to time and they look gorgeous in 1080p. The screen is fairly close to ones eyes so it really looks like a 40-50" depending on how far it is away from you. Since the pixel density is high videos are beautiful.
Both iPhone 6 and iPhone 6 Plus is noticeably bigger than the iPhone 5 being too small to enjoy movies.
I have the iPhone 7 Plus and iPhone 6S.
However when you have an iPad, iPhone 6 Plus is not big enough unlike the iPad Mini/ iPad Air and iPad Pro.
1. iPad Mini 7.9 inch is about 2.4 Times bigger than the iPhone 6 Plus.
2. iPhone 6 Plus when watching movies even at 5.5" is just 2.2 times bigger than 3.5 inch iPhone.
iPad Mini and iPad Air can replace laptop for watching movies as they are closer to laptop size than Phones.
While iPad Pro 12.9 Inch completely replace HDTV FLAT TV television since iPad Pro is more portable
My 6S Plus is the biggest phone I have ever owned and I must say watching full length movies is quite enjoyable compared to the smaller phones I'm used to.
Not as enjoyable as my iPad mind you, but for the portability of an iPhone I can't complain.