The open source firmware does not run on the WRT1900ac yet as there are issues in the wireless drivers.
To the questioner though, I recomend 802.11ac over 802.11n. Myself I'm using ASUS RT-68Us for my network, good hardware and the stock firmware is actually pretty good but they support openwrt tomato and dd-wrt too. There is also a NETGEAR box it that that has better hardware and supports the open source firmware but apparently the stock firmware is pretty bad. However compared to WRT54-GLs these routers are about twice the price.