What this means is don't be too picky about deciding you don't like HTC software for example, if the actual hardware is ideal for you because you'll be able to change it. I couldn't stand Samsungs software so on my Samsung handsets I changed the launcher and it may as well have been an unbranded phone then as far as the software was concerned.
True, but also not.
Personally I do not like Samsung. The S7 and Note7 are the 'best' phones in the world on paper, yet sometimes still lag in basic UI cause it's so poorly written and full of bloatware.
Launchers will change the look and some functions, but not improve that performance. For that you'd need to flash custom ROMS, but you can't do that on any of the Exynos based Samsungs.
On most other brands like HTC, LG, Lenovo/Motorola, Sony, Huawei, etc based on the Snapdragon SoCs you can flash a custom ROM, but launchers would also suffice cause the software is a bit better written and they have less bloatware apps (especially that HTC 10 cause they actively avoided duplicate apps from Google).
Samsung are market leaders cause they were among the first to compete with the iPhone at a lower price and people will just buy what they are used to. I used to work in a phone store and people would go "Yeah I'll have another Samsung, I liked this one a lot" and while I was setting up the contracts they'd constantly complain about how they really needed a new phone cause the battery was crap, Touchwiz kept dying and and it was slow as hell...
About the size of the phone; I would't worry about going 5 or even 5.2" despite the iPhone 6 having a smaller screen. iPhones have a LOT of bezel going on (about 35% of the front of the phone) so and Android phone with a larger screen will generally be only ever so slightly bigger, if not the same size. The best thing you can do is going into a physical store and try out some phones by holding them in your hand. See if you can touch the top left corner without changing the position of your hand to much and all that.
Another thing about Android is that, like others have said, it has a LOT of functions. And sometimes to accomplish smashing all these functions on your screen I feel the design suffers a bit. For example one thing that annoys me a lot is the bar of recommended words above the keyboard; When I type a word wrong and then go back to it by tapping on it and picking the right option from that bar, it's very difficult to actually click that word in the bar cause the UI displays a little pointer over it (in case you want to change the position of the 'typing cursor' within the word). Some things are just compromises and not all of them will be for your personal benefit.
If you like iOS, Xiaomi offers a pretty similar UI/UX to it.
Good shout. Huawei do the same.