The simple yet true answer is, I believe, "the one you're most comfortable with". They're all good for song accompaniment, though the "simplest" or "easiest" types of accompaniment will likely differ from one to another.
I myself have used both anglos and duets for song accompaniment (among other things), but far more I use the English, because that's the system that I've always found most comfortable. There are others who also find the English to be most comfortable, but still others who find it terribly uncomfortable. Likewise for the anglo and the various duets, some folks find each of them very comfortable, while others feel the opposite. And there are yet others who seem to find them all reasonably comfortable.
There's not much point in trying to make good music if it's a pain to do so. On the other hand, if the playing itself is enjoyable, the music probably will be, too.
I think it would be counterproductive for you to dilute your learning efforts at this stage by trying to work with more than one system before you've reached reasonable proficiency with even one. It sounds like you're already comfortable with the anglo, both physically and musically, so I suggest you continue with it unless and until you find yourself frustrated in trying to do something with it that you just can't seem to get. And even then, with a bit of help from your friends here, you might find yourself able to do that something, too.
There will always be the option to experiment with other systems in the future, though if you're comfortable with the anglo, you're not likely to give it up, but rather to add another system. Regarding "room for improvement", there are many factors involved, including relative sizes. E.g., a 30-button anglo can do more than a 20-button, and a 38- or 40-button can do even more, but each step up gets heavier to hold. In this post in another thread, Geoff Wooff notes some factors with regard to the Hayden layout. So how much "improvement" can be made on any of the keyboards and how much work will be required to make it, will depend both on what you want to do and subtle details of how the keyboard sits under your fingers.
A few encouraging examples in favor of sticking with the anglo even if you want to get "fancy", as long as it feels "natural" to you:
Harry Scurfield played 40-button anglo with the band Bayou Gumbo (1987-2012), doing "cajun, zydeco, reggae, blues and high life amongst others." And I remember one session where Harry started playing Frankie and Johnny in the key of C# on his C/G anglo, but then modulated into D when a couple of us gave him pained looks.
Zak van der Vyver, who grew up in the Boer tradition, does rich jazz-flavored stuff, also on 40-button anglo.
John Kirkpatrick is well known for playing all sorts of stuff on various anglos as well as on the melodeon.
But 40 buttons isn't a requirement. I once heard Scott Joplin's Maple Leaf Rag played on a 30-button anglo, and it sounded as if it had been composed for that instrument.
There are other great examples (as well as many not so great) of players and playing on YouTube. I'm sorry that I haven't kept a proper list, but I'm sure others here can point you to their favorites, and you should easily find more worth hearing than you have time for listening.
very helpful post so whats the advantage of an anglo over a duet?