If you consider between these 2, I assume that you are OK with both size, so I take that out from the equation.
So now, for 6+:
1) Has better battery - for normal usage (not 3D game etc), it's about 14hr vs 11hr usage time available. Should be enough for one day use, but the 6 may be a bit tight if you use the message apps a lot. For phone call, it's 24hr vs 14hr, a huge difference (but again, either case is more than enough for a normal user). For standby, 16 day vs 10 days (most likely you will charge it every night anyway). I am now with the 6+, and never need to worry about the battery, on a heavy day (17+ hours standby time and 11+ hours usage time, my phone still has >35% battery).
2) A better bigger full HD screen - zero distortion play back when you replay any full HD video which was token from the iPhone. Also better contrast on 6+, 1400:1 vs 1300:1.
3) Have OIS - this is important if you will take picture in dim environment. I can take pictures on a boat at night time, which the image still sharp.
For the 6:
A) Relatively better graphic processing power - same GPU, but around 50% less pixel to drive. Which may give you better gaming experience on the iPhone).
B) More storage - I am not considering myself a heavy user, however, I already used up 65G of memory on my 128G 6+.
C) Less software issue - the 6 has the same 326ppi as the 5 or 5s, but 6+ is 401ppi, which is new for iOS, that means more potential compatibility issue on 6+.
D) Lighter - if this is a part of your consideration.
If I were you, I will make consideration by the following order.
Is 64G enough for me? If no, then go for the 6 (be careful, if you want to take 1080P 60PFS video on your iPhone, that will require lots of storage).
If yes, then is battery very important for me? If yes, go for the 6+ (e.g. for me, I am not working in the office, I can't re-charge my phone in the middle of the day, so I go for the best battery option)
If no, do I care about the camera performance? If yes, go for the 6+.
If no, do I care about the extra weight? If yes, go for the 6.
If no, do I want a better screen? If yes, go for the 6+.
If no, then go for the 6 with potential less software problem.
Anyway, here is a picture took from my 6+. As I said, it's at night time and on a boat. I just did that by hand, no tripod, no post processing. I don't think it's possible to have this quality without OIS.