Q: On connectivity selection of multiple wifi routers in the same LAN We got an N300 Fiber Optic Router (default hardware from ISP) and the Fiber download speed is 200 Mbps. Given that the router is just N300, we couldn't get that 200 Mbps. So we want an AC router to get that speed in wifi. 
Questions:


*

*Suppose we get ourselves an AC1900 wifi router, and use a LAN cable to connect to the N300. If I am in a wifi intersection of these two routers, are the two routers smart enough to use the best connection available at the given time (in this case, AC1900 will be used in most cases rather than the N300 unless the AC1900 is very congested)? 

*If in the case that the AC1900 can spread wifi farther and cover the whole area of N300, is it recommended to turn off the wifi of N300 entirely?

*For 200 Mbps Internet speed and to be futureproof in the long run (ex. the ISP increase Internet speed for free), is an AC1900 router theoretically sufficient or should I go for higher number? We got about 10-20 devices in our LAN.

A: For Question 1, it isn't up to the routers. If more than one 802.11 AP in range is publishing the same network name (with the same security mode), the 802.11 standard leaves it up to the client to decide which to join. It's even left as an implementation detail; the standard does not specify how the client should decide. In my experience, most clients just use the naive algorithm of joining whichever network has the stronger signal.
So, in answer to Question 2, I'd recommend turning off the N300's Wi-Fi if the AC router can cover the whole house, so that nothing accidentally joins the N300. I suppose you could leave it enabled, but give it a different network name so you don't join it accidentally.
As for Question 3: Routers with higher numbers than 1750 aren't necessarily worth the extra expense. Going from 1750 to 1900 means it supports a nonstandard 600Mbps mode in 2.4GHz, which few client devices support (and why would you bother when you can do 1300 in 5GHz?). Going above 1900 either means it supports 4x4:4 (4 spatial streams) rates, which most clients don't support (most AC clients only support up to 3 spatial streams), or it means it has a second 5GHz 1300Mbps AC radio. The second AC radio is usually billed as allowing you to put your good/fast 3-stream AC stuff on one radio on a good channel, and put your lamer 1-spatial-stream (or 5GHz N or 802.11a) devices on the other radio on some other channel, where the slowpokes can't waste the airtime the fast devices want to use. These are called "tri-band" routers (even though two of the radios work in the 5GHz band).
I could see some sense in getting a so-called "tri-band" router in order to keep the slowpokes out of the fast lane, but I recommend against the models that do 4x4. It seems to me that 4x4 is failing to catch on in the client market, because with the trend toward aluminum, thin & light laptops, there's just not enough room inside the laptop for 4 Wi-Fi antennas (plus Bluetooth, etc.). I think the main path the industry will take toward higher Wi-Fi speeds will be 802.11ax, not 4x4 AC or 802.11ad. So don't bother paying for 4x4 unless you buy tank-like bulky "gaming" laptops, or you have a desktop PC you don't want to pull Ethernet cable to.
