I've got a D/G Erica too. Mine is rather battered and 2 treble notes are reduced to single reed. I got mine for Morris dancing but I dropped that and the box in favour of an Irish session I found and mandolin/tenor banjo. I still get my Erica out occasionally but, while I can get some simple Morris tunes out of it, I never got to a stage where I could use it in an Irish session.

I got a B/C box fairly recently as I'd long fancied a try with one. I started with good intentions but it's dropped to somethng I really must put an effort into one day.. I think if you have played tunes in the given key on a harmonica (in my case I had a couple to play with for a while in childhood), playing the straight rows on a D/G instrument will feel quite familiar. Playing say a D scale on a B/C just seems weird to me at the moment.