Please help me choose (6s vs 6s plus)

I've been using iphone 4s 16GB for the past 3 years and upgrading to iOS9 was a big mistake, everything is slow as **** so it's time to upgrade to 6s.

16GB seems enough for my current use but if I am to invest in a phone for another 3 years I might consider buying the 64GB version.

Here's what I use my phone for:

messaging (a lot of it, I got all kinds of messaging apps) and mail

Internet browsing (a lot of it)

news/rss/blogging apps

maps (I get lost a lot)

music / music streaming (eg Spotify)

social media (fb, twitter)

average camera use for pics only

organising apps (calendar, notes, lists, reminders)

Here's what I don't care about:

Video shooting

Video watching

Gaming

Document editing

I would also like to be able to read a book or a pdf document easily on my phone which is not the case with the tiny display on 4s. I hear a lot of people saying that iphone 6 plus has replaced their tablet and given that I don't own a tablet but I need one that's something to consider (I don't like the idea of owning many devices that do virtually the same thing). At the same time I keep my phone in my pocket all the time since I move around a lot with public transportation and I worry that 6s plus will be a pain to carry around.

I'm on a tiny budget but I do plan to invest on a good phone since I also use it for work a lot. Given everything that I've described which version of iphone would you advise me to get?