Keep that GL, it's far better, however don't flash it unless you intend to use it for a long range WLAN as some suppliers will have serious contention about swapping it out should it have been previously flashed.
If you decide to use the router to connect to the mesh, you would need another one for your internal network, as the one connected to the mesh just becomes a wireless bridge.
Although I agree, the Linksys is a very cheap and good platform for mesh connectivity it's by no means the best. I would always recommend a Mikrotik RB112 with a CM9 radio card. (that's another story though).
I would recommend getting an ADSL modem to go with that WRT because it's got some really nice functionallity even with the default software installation.