IPhone work smooth with only 2GB ram dude. I have an low end android phone with 1GB ram and it works very smooth. Also I have Sony Xperia Z3 and it works better with its 3GB ram than Samsung Galaxy S7 with its 4GB ram. It is because of perfect software optimisation.
I’ve used Vivo with 6GB ram. It lags many times but my Z3 never lag. In one word - if ios go for iPhone - if android go for Sony or HTC. Samsung and LG = never.