I would be of the opinion that the hardware differences between the two *are* minimal (aside the obvious screen and physical size). In day to day use I doubt I would notice the difference between the different PPI of the screens, and OIS isn't going to suddenly make a massive difference to the vast majority of photos I take with my phone. In fact the main aspect of the plus that had me considering going for that rather than the 6 was the greater battery life, but as I rarely have issues with the 5S and the standard 6 is as good or better than the 5S anyway it wasn't really a good enough reason to switch to the plus.
A few months back I bought a Moto G just to dabble with Android after we got my mum one and I then decided to try android "properly" and got an Xperia Z2 to try instead of my 5S. After a couple of months I had decided I liked the idea of a slightly larger screen, but that 5.2" was too big for me, and 5.5" certainly would. That along with some other software problems I was having with the Z2 ended up with me selling it on and going back to my 5S until the new iPhone came along (which was already strongly looking like being 4.7" & 5.5").
The 6 and 6 plus are certainly far more closely related than the Galaxy "full fat" and mini variants are, though I believe the S5 mini is somewhat closer than previous mini versions. I think Apple have tried to avoid "forcing" people to go for the larger (more expensive) phone in order to get the top of the line by not making too many hardware differences between the two. Sure there are still some differences, but you still get 95% of the improvements over the 5S by going for the 6 rather than the 6 plus. I think they have got the balance right and I for one will prefer the more compact form factor than having OIS, a marginally sharper screen and extra battery life.