Hello  @Ammmmr7 

Thank you for being interested in TP-Link range, extender, here is a comparison of the two models spec which might help you  tomake the choice:

https://www.tp-link.com/en/compare/?typeId=10&productIds=45282%2C58539

The main difference is that RE605X supported the updated Wi-Fi 6 standard, which can get 1201Mbps 5Ghz link speed with 2 *MIMO (antennas)

While RE650 supports up to 4 MIMO and maximum 1733Mbps 5Ghz link speed ; besides  RE650 starts to supporting OneMesh feature since RE650 V2.

In general, if your router is of WiFi6 or has a plan to upgrade to Wifi 6 router, please consider RE605X; if your router is of high-end and support 3 or 4 MiMos already, RE650 would be better, hope this helps.