The newer ASUS (RT-N12/16+) use a 2.6.x kernel (and actually don't work with the old 2.4.x kernel) on tomato, and given now, I'd get the RT-N16 running tomato. My RT-N12 (I couldn't afford the more expensive RT-N16 at the time) is fine but only has 4MB of flash, so I can't put the "full" tomato on it, instead I compile it from source for a thinned out build that gets rid of a number of things yet keeps both IPv6 & OpenVPN.
I haven't tried either of the Buffalo routers, but my budget ASUS has been rocksolid and works very well.
A word of advice about tomato though, the development has stalled somewhat and led to the continuing development of a number of "modifications"(1) developed by different people, where each goes its own ways (e.g. Shibby, Toastman etc.). They can be found via the sub-forum and continuously add new features, improve existing ones and also fix some bugs etc.
I haven't tried either of the Buffalo routers, but my budget ASUS has been rocksolid and works very well.
A word of advice about tomato though, the development has stalled somewhat and led to the continuing development of a number of "modifications"(1) developed by different people, where each goes its own ways (e.g. Shibby, Toastman etc.). They can be found via the sub-forum and continuously add new features, improve existing ones and also fix some bugs etc.
(I am using Toastman's tomato no-usb mod)
(1): http://tomatousb.org/mods