According to this article on about.com:
Definition: 802.11n is an IEEE industry standard for Wi-Fi wireless
local network communications, ratified in 2009. 802.11n is designed to
replace the older 802.11a, 802.11b and 802.11g Wi-Fi technologies. Key
Wireless Technologies in 802.11n.
802.11n utilizes multiple wireless antennas in tandem to transmit and receive data. The associated term MIMO (Multiple Input, Multiple
Output) refers to the ability of 802.11n and similar technologies to
coordinate multiple simultaneous radio signals. MIMO increases both
the range and throughput of a wireless network.
In short, the 802... protocols have not much to do with the Antenna (there are no individual Antenas for each protocol), but the frequency does. The 5Gig does not have wide range then the 2.5Gig, it only allows faster data transfer.
To make it simple. Your Internet Service Provider (ISP) sets your MAX internet speed, but your internal network(computers/tvs/and whatever else you network) has its own speed.
For Internal traffic faster is almost always better. If you have lots of Internal traffic using the router (gigabytes of data transferring) from one point to another in your home them using the faster router is suggested.
So if you do not need the extra features then the Airport Express will serve your need.
For the price of Airport Exterme ($199) you can get 2 Airport Express and use the second one as Range extender as ultimate solution.