To me, I would rather buy the largest model that you find on most boats. As already mentioned, you can adjust the maximum charge rate to match your battery bank. Output is controlled by how many devices are used at the same time. Going with the larger size also means that you can easily increase use and charging should you increase the battery bank size. The other consideration is that the larger capacity can be used underway as you can utilize both the bank capacity and the alternator capacity. So as an example, if you want to cook a Turkey in the electric oven, you might not have the battery capacity to run the oven through the inverter, but the added amperage of the alternator allows it to happen. When the oven reaches set temperature, when it cycles on and off, the alternator is recharging / maintaining the batteries.
I've had great results with my Magnum Energy 2812 pure sine wave and highly recommend it.
Ted