If you have AC1900, I wouldn't spend any more. If you are close enough to REALLY benefit from WIFI AC, then you can probably use wired which will always be better IMO.
I don't own any of them. AC1900 is the price performance sweet spot. Unless you are in the same room as the router, AC WIFI may not benefit you. Are you currently able to use 5Ghz?
Yes I am. I have the wrt1900acs currently but I'm wondering if I should return it and get the asus 3100 ac because of it's mu-mimo function. I'm not sure though.
If you have AC1900, I wouldn't spend any more. If you are close enough to REALLY benefit from WIFI AC, then you can probably use wired which will always be better IMO.