Which wifi extender is better overall? EX6200 or EX7000?.

The EX7000 supports higher, maximum link speeds at 5 GHz (1300 Mbps vs 867 Mbps), though this may not necessarily translate to faster speeds in real life because it requires 3x3 stream support in your devices.  3x3 stream support, while not rare, is relatively uncommon, found only in high-end laptops such as the Macbook Pro and desktop machines with quality Wi-Fi adapters.  Most smartphones and laptops are 1x1 or 2x2 stream devices.  The EX6200 can accommodate 2x2 stream devices.

Another difference worth noting is that the EX7000 can be used as a wired Access Point.  That is, it can be connected to the main router via an Ethernet cable.  This allows the Wi-Fi radios to be dedicated towards servicing clients rather than splitting duties communicating with devices and the router.  If your Internet service is, say, 50 Mbps or higher, than a wired Access Point will increase your chances of fully leveraging that bandwidth through the extender.  In contrast, the EX6200 cannot be used as a wired Access Point.  It must be used fully wirelessly to communicate with devices and the router.

Apart from that, the differences between the models are marginal.  The EX7000 has a slightly faster dual-core, 1 GHz processor, while the EX6200 appears to have a dual-core, 800 MHz processor.  I expect the range to be similar for both models.  I think I read somewhere on this forum that the EX7000 does not have beamforming, while the EX6200 does.  Beamforming can help with speeds.  I don't have any data to quantify the difference.