Hi Ian,
There is not much difference in low light performance between the two so you could either get the 60da for some extra hydrogen alpha sensitivity. If you want really good low noise in the canon brand the 6d is where it is at and will no doubt keep you covered for ages to come - you will have no issues there.
however as you say portability is an issue for you.
http://www.dxomark.com/index.php/Cam...(brand3)/Canon
cheers,
rusty