This is really difficult, because the instruments are so different. Even the three concertina systems are really different instruments.

I disagree with Marje about melodeons. Plenty of people use melodeon very effectively for song accompaniment. However they mostly use "quint" boxes (eg D/G, C/F etc) rather than half-step like the B/C you tried, which are mainly found in Irish music where they make little use of the basses (someone will now come up with a long list of half-step singers!)

Having limited keys isn't a problem unless you sing in a wide range of keys, which most people with untrained voices don't. As long as you find one in keys which suit your vocal range, you should have most of the chords you need for those scales. You can also play chords on the right hand.

Phil Williams mentioned that many people sing in F, but didn't point out that this is a really good key on a C/G anglo.

The English concertina is good for song accompaniment and gives a full range of keys, but you have to be happy with the way the scale switches from side to side, rather than adjacent buttons. It's particularly good if you play from written music, but it's very logicality makes it less intuitive (at least it does for me).

Duets may give you the best of both worlds, but some are more complicated to play than others as you may need to learn completely different fingerings for different scales. The Hayden system overcomes this.

The important things are:

does it have the sound you want? If you've been attracted by the sound of the concertina then a melodeon won't do, and vice versa

do you feel you'll be able to come to terms with the instrument to learn it well enough? For some people, the push-pull of melodeon or anglo is intuitive, for others it's baffling. I have no problem with push-pull, but get muddled with the way the scale alternates from hand to hand on an English concertina. You must find an instrument which suits you.

Finally, bear in mind that you still need to give most of your attention to the song, rather than playing the instrument. You need to be able to play the accompaniment well enough to do it while thinking about something else! It's essential that you're comfortable with the instrument you've chosen.

No one can really advise you on this, all they can do is point out the pros and cons of the different instruments. You have to decide what's right for you. Take your time to try out the different options.