First you get nowhere near those magic numbers in real world. Most times to get over 100mbps you need 802.11ac devices.
The main difference between those 2 is the 300mbps one attempts to transmit 2 different overlapping signals on the same radio channels. They do surprising good job of extracting the data when you consider they are intentionally sending singals that interfere with each other. As you move up to say 450 or 600 going to 3 and then 4 signals they are less successful so the amounted added each jump is much less.