AirPort Express and Extreme Wi-Fi strength comparison
Alright, this is an easy catch here.

If you compare AirPort Express which has 2 built-in antennas , the AirPort Extreme has 6 built-in antennas. You should be able to do the math here and jump to the conclusion that the Extreme would have a better range and signal because of this fact.

To see the source, please click the back url below the Airport Extreme picture here.

Please check this comparison model provided by Apple.
The difference for you 


Express max 300Mbps
Extreme max 1.3Gbps