20 settler increase is a pretty big stacking addition early on. I'm used to seeing about 45 base settler values, often dropped due to jungle or arctic for the early regions. Going from 35 to 55 is a massive increase in colony setup speed. More/faster colonies easily trumps better colonies. The alternative choice, for me, is removed attacks, as this allows me to either/both of keep my entire army at home and keep army maintenance off. The cost of simply keeping that slider up to garrison a colony is often more than the maintenance cost of dropping an extra colony, plus the opportunity cost of diverted FL towards that land.
I would probably need to figure out the actual math on increased production from assimilation; I'm sure there's a breaking point where the % gained from that outpaces the % increased growth potential of the flat settler increase, which one would expect when comparing a % to a flat bonus, and with high enough dip tech (and stuff like Iberian national ideas) you'd grow faster with a "tall" assimilated colony over a "wide" settler increase. That said, if you are staring at a bunch of 1/1/1 areas with thousands of natives each, assimilation may work out early, whereas stuff like the Caribbean's 8-10 starting development base island chains with only like 2500 natives tops it will not.