@RuiS 

Hi,

Do you have specific performance numbers in mind that you are looking for on the upper floor?

One major issue with Powerline adapters is (and I mean all Powerline adapters of any manufacturer) that it is nearly impossible to predict the resulting performance in advance. (i.e. before the purchase)
Under certain conditions the achievable data speeds might end up being really slow.

Another thing to consider is that Wi-Fi equipment made for stationary use is usually designed to spread its wireless signal in mostly the horizontal direction. (like in the first picture)
If the Wi-Fi equipment has adjustable antennas, then one can try to optimize the signal by adjusting the angle of the antennas. (like in the second picture)
The TL-WA850RE obviously doesn't have adjustable antennas, it could only be the tilted as a whole if the circumstances permitted.

For the range extender it makes no difference whether it connects to a Wi-Fi router or a Powerline adapter with Wi-Fi or something else that broadcasts a Wi-Fi network. (provided the devices conform to the official Wi-Fi standards)

Note that the drawings below are only a rough illustration. They are not an exact representation of how the Wi-Fi signals spread. 

  @RuiS 

How about this? Just put the WPA on the second floor, and RE on the first floor, which could avoid the situation what @woozle said about the signal attenuation through the floor between the RE and Router. 

  @Dooooo 

My router has no antennas, so cannot make any adjustment as woozle suggested and the solution of RE upstairs and WPA downstairs, I confess it had already crossed my mind.

The problem is that prior to purchase it is impossible to make any trials, obviously. So, I think I will risk buying the TL-WPA4221 and then try both ways to see the one that gives better results.

Thank you both for your support.

  @RuiS 

A paradox I think so. You won't know it before you buy it and vice versa. Although many things works like this. How about take a try. If it's not satisfying, just return, if egilible.