Help choosing a wifi extender

My cable modem (Netgear CM500V) and wireless router (Netgear R6200 Smart WiFi Router) are in the basement.  I currently have a Netgear WiFi Range Extender Model EX6100 on the floor above the basement. I get good range in the front of the house where the extender is, but hardly any signal at the back of the house, and none at all on the back patio.  I thought adding another extender in the back of the house might solve this problem.  My question is should I order the same model number I have (which doesn't seem to be available on Amazon), or should I order Extender Model EX6150, which seems to have a longer range?  I'm assuming that I can piggy back one extender off of the other, rather than trying to get the signal from the wireless router in the basement, but I'm not sure about this. Also, if is better to get the Model EX6150, does it matter which is closest to the basement router? i.e. Should the EX6150 be closest and have the Ex6100 piggy back the signal from it, or visa-versa? I hope my questions make sense. I just don't want to order the wrong extender.