This reminds me when Sony released two models of their PS3 back in 2006. Or how the PS Vita went slimmer but still got compromised with cheaper build quality. Or the smartphone flagship mini versions that generally get compromised vs their bigger brothers. I think Apple should make 32 GB the standard. But have 64 GB 5.5 at $299 and 128 GB 5.5 at $399 with contract.
It isn't even about picking between 4.7 vs 5.5 anymore. To me, it is picking which one is the better version of iPhone 6. Bigger footprints means you can cram more things into it. Faster CPU, bigger display, better camera, 2910 mAh battery, Sapphire glass, more iOS8's multi app window purposes, 128 GB option or a smaller device for better one-handed/pocketable use? I find very little advantages from the 4.7 model. This mini model got pigeonholed to the Pee Wee league for people with small hands or too scared to adapt carrying a larger phone with better specs and longer juice time. I see girls with Samsung Galaxy Notes no taller than 5-ft use it with no problem. And phab iP6 should be popular to them since they have purses/bags and older people with weaker vision. It also depends on which smartphone was your most recent the same way people try to adjust from driving a sedan to SUV and then back.
As seen from LG, a 5.5-inch can be comfortable if done right with little bezels and rounded edges and rear. My Mi 3 is nearly the same dimensions as the G3 and I find it more comfortable to hold than my 4.7 inch HTC One because of the rounded edges, slimmer profile, and very little overheat on battery saving mode (tops at 101 degrees vs M7's 111 degrees after 20 min of PvZ2).
For ideal one-handed use
4.7 > 5.5 inches
Better iPhone 6 model
5.5 > 4.7
For me, I prefer the s model. So iPhone 6s (phab) if I ever returned to iOS next year.