I have C3700 N600 and want to extend range

I moved to a bigger place and my model+router is not able to give me good signal around my place.   I have been debating on wether to get extenders like (NETGEAR WiFi Range Extender EX6120), can you please suggest if going and buying a new router would be good or extender would be ideal. 

I use comcast xfinity : if I go and buy a new router like the one (NETGEAR Nighthawk Smart WiFi Router (R6900P) - AC1900 Wireless Speed (up to 1900 Mbps) | Up to 1800 sq ft) - can I use existing N600(C3700) as my modem and turn off the router feature in it or do I need to explore on other options ?

Thanks in advance :)