There are a number of people who can play the Anglo well in different keys (I remember Brian Peters accompanying Lou Killen on "Blaydon Races" at a folk festival a couple of years ago, seamlessly moving from G to F when Lou wanted to sing a bit lower) but for the most part on a C/G instrument, say, you want to be in C or G, maybe Am, Em or Dm.
Unless you can afford a lot of instruments (and concertinas do _not_ come cheap) you might be better off with the English if you like to sing in a range of keys. And carrying a large number of concertinas around significantly reduces the portability index. (That's primarily why I gave up on melodeon, the other reason being it's a bit like playing the left hand side of the anglo with the right hand).
Good luck.