There's a lot of bloatware non-crucial to the phone that cannot be uninstalled through conventional means (including carrier-specific bloatware), and it obviously takes up a some internal space. And then there's the pointless and silly hidden bloatware services that spontaneously run in the background which are notorious for eating up RAM. Even 2GB of RAM is not enough for a phone these days....
My Galaxy S5 currently idles with nothing running at
only 400MB of usable RAM left for applications and multitasking (out of the 2GB). This is a stock ROM, but I can definitely tell you that the RAM usage is much lower on a custom/modded ROM.
Not too keen on Apple's iPhones really. I had the 6 and the 6S, but the greatly limited app store and the app choices compared to Andriod back when I had both the phones, and the lack of customization for IOS plus no expandable storage (and the expensive memory options), lack of removable battery and lack of water proofing combined I kinda hated it because it wasn't worth the high price of the phones.
The build quality is among the best but, the full body aluminum isn't practical for me as it just makes the phone heavier. And if you buy a case to protect the phone anyway, it makes the aluminum pointless with the phone being even more heavy. Not to mention the body to screen size ratio on iPhones have never been the best, compared to say, LG's Optimus/G series.
Supposedly the iPhone 7 might come out with the headphone jack omitted too, which is really not a consumer friendly choice if they force us to use an additional adapter all the time, or use bluetooth and battery powered headphones which I haven't heard of any that sounds great yet..