Are you looking to hatch once or twice a year- or more often? Do you want to be able to have multiple smaller settings going at once? Duck eggs do better being laid on their sides in an incubator so thats something to consider if you are needing something that will turn the eggs automatically.
Last year I upgraded my incubator and considered many points when deciding the best model for what I needed the machine to. I hatch year round- so it had to hold temperature very well - especially over the colder months, ( something the foam bators are terrible at. ) have a roll turn mechanism, preferably have a tray for hatching - which can increase the space available for incubating if needed. The two models I came up with were the Rcom 50 and the IM ( which is not availble in the USA unfortunately- but there are other similar cabinet style ones availbale ) In spring and summer I hatch larger settings and make use of the hatching tray - I rotate the eggs between trays so that they all get their turn being hand turned 3 times a day- or roll turned automatically every four hours.
This is my incubator- the left hand side has eggs half way through incubation- and in the middle- the ones just put in 3 days ago.
The trays are removable for ease of cleaning and for candling the eggs. I pull the draw out about 1/3 of the way- leaving the eggs in place I can put a small flashlight to the end of each egg without having to handle the eggs. The time can be set digitally for how often it turns the eggs - from as little as 1 minute-through to 9 hours hours. One of the best things is the clear door- you can easily see the eggs and the hatching trays- great for when they are actually hatching.
The bottom tray is out in this pic- since I just finished a hatch over the weekend I took it out for cleaning. You can see the size of the water tray- which is enough for incubating and hatching alike- just a small turn of the air vent in front will raise up the humidity.
This is a link to show the second incubator I considered. In the end my decision was based on the size and the added bonus of the hatching tray in the IM. This one is fully digital - fairly compact in height - but the other dimensions are quite large. It also has pre programmed settings for hatching different kinds of eggs.
http://www.rcomincubators.com/product/categoryid/124/productid/1503
They are both in the upper scale of cost- but with incubators- you really do get what you pay for. A foam bator is a good start up point or for people doing one or two hatches a year- but for something for more frequent use- or for harder to hatch eggs- its well worth paying the extra to get the higher hatch rates possible with a better machine. Local weather conditions can have a big impact on what kind of a machine may be better for you - as can where you plan to keep the incubator. If you plan to keep it outside- making sure you have something with a very reliable thermostat is very important.
The foam type incubators will only hold around 30 duck eggs laying down - more if they are a bantam breed.
Sorry for this being so long...I just think its really important to choose the right one. Finding out you have wasted money when you have repeated failed hatches is a hard way to learn. There are so many variables when it comes to incubating, Starting with the best machine you can make a huge difference to how many gorgeous fluufy butts you end up with after the long 28 days.