Quick look:
The first one does something like link aggregation on both 2.4 and 5 GHz bands at the same time. To work properly, the extender and the router would meet to support that method of link aggregation.
The jackass in me says the better solution is to run a Gbit wire between areas served, and use something like ddwrt or openwrt to make them play nice together, that way they aren't capitalizing on the spectrum just to stay connected to each other.
See, when you bridge directly over the wireless signal, the device talking to the repeater has to wait more for the channel to become clear, because its message is then broadcast again to the main router.
When the extender and the router are joined by a Gbit link, the message from the extender to the router goes over that wire instead, so the network is more responsive.
As for why one would cost more than the other? The one doing aggregation does .ac while the other does not. The link aggregation is a software thing, and should not really cost more IMO, but that's life.
Personally, I would get another router that has ddwrt or openwrt support and put a single Gbit Ethernet patch between them, and set the new device as an access point.
the only time i would consider it necessary to bridge over wifi like that is if you are trying to extend somewhere a cable cannot be routed. say the guesthouse in the back yard, or in my case, extending my wifi to my elderly neighbor's house. (she cant really afford internet, so i help her leech mine.)