An external AP would give you access to 802.11n, that would be faster yes, but not necessarily more stable. An Alix with an Atheros 5413 card is perfectly stable, and it supports 802.11a wich is often used for wireless bridging because it usually have much less interference. It ultimately depends on how much speed and flexibility you really need.
It's inconvenient to use AP wired to alix ? i think is a better solution in comparation with wifi cards, you also could use 300mbs acess points and on those to use antennas.From my experience will be a better solution and more stable, i'm using pfsense but on pc platform with AP on lan AP is exactly your specified routers from above.