I take both. I have a smaller iphone (a SE which is the same age as a 6 but smaller) and a normal ipad. I have never traveled for 5 months but don't see the length of a trip as a big factor.
I use the phone for its usual purposes - calls, texts, internet when out and about, maps, camera sometimes (I am pretty into photography and bring my DSLR but occasionally the phone is easier).
I use the ipad for moving my photos from the DSLR. I can store and edit them on it better than on the phone - the screen size is very important for this. I have the photos saved on my memory cards, on the ipad, and backed up to the cloud whenever I find internet that is strong enough. I also prefer to read on an ipad vs. my phone for the size factor, so I will load my ebooks on both it and my phone. Also, I find web browsing with more reading/parts easier on the ipad so at night I will use it for my research for the next day (checking opening times, reviewing wikipedia for the history of a sight, finding potential restaurants, that kind of thing). Facebook and such is fine on the phone, though. Finally, I always load a few movies or tv episodes on it. I don't watch much when traveling but it's good to have on the plane or train.
Honestly, if it weren't for my photography interest I would consider not bringing it, too, but it comes in handy and I think I'd miss it. I don't carry it with me on a day-to-day basis - it stays in the hotel room, either in the safe or buried in my suitcase. I use it only in the hotel, airport, and on planes/trains.