JohnnyForeigner refilling a single boiler machine soon becomes a habit and personally I don’t think it is worth choosing a different machine based on this. If you’re doing espresso then it should stay full (water pushed in pushes water out). It’s only when steaming that you have to fill afterwards, as the water is being boiled off.
If you’re not doing back to back milk drinks I think a single boiler is a good choice. They’re cheaper to buy, smaller, easier to descale and more energy efficient. For steaming you’re heating 300ml from around 90°C to 140°C, whereas most double boilers you’re heating 1L+ from room temp each time. Even a HX like the Mara X you’re heating 1.8L to 125°C.
They’re just a bit slow for multiple milk drinks, as you can’t steam and brew at the same time, and there is a lag to steam temp, and then to brew again you have to flush water through until the temperature is back down to brew temp. I don’t regret getting a single boiler. It’s perfect most of the time. Even for 3 flat whites it doesn’t take that long, I tend to pull the 3 shots in a row and then steam the 360ml of milk I need to pour three flat whites. All in it probably takes me 5-10 mins, not a deal breaker.
Like all things, your use case and preferences will ultimately decide what is right for you.
Enjoy the shopping (and deliberating).