Other than about $800? Just kidding, I'm going to guess that the pump is the main difference. As far as I can tell, and I'm pretty much a newbie to waste oil heating, there really isn't much 'new under the sun' in burning waste oil. Most all of the designs that work use a siphon feed, either from a constant level tank or a pump. Physical layout of where you want to use it will have a bearing on what you want to use, for me a tank wasn't really handy. Most people on here and on altfuelfurnace on yahoo groups seem to think the metering pump is the best way to go if you can afford it. The nice thing about the Clave pump is that it's adjustable, and with a metering pump the temperature or makeup of the oil doesn't effect the flow rate. I'm currently using a pump but with pressure regulators, and it does require a little tweaking during the day as the oil flow rate changes as it warms up and thins out. I'll likely go with a metering pump eventually myself, but a lot of the 'factory' waste oil burners do use the pressure regulated pump like I'm using now (mine is from a Lanair furnace). From reading the Clave advertisement on Ebay, I do like the fact that the oil is not heating when it's on standby, but right now my setup is the same way, more by accident than design. That should make for less buildup in the block, although again, there are factory furnaces out there (my buddy's Cleanburn for example) that run the block heater anytime the furnace is powered. He doesn't seem to have any problems with his, and he bought it well used and has used it hard for three years now.
I suggest you talk to Craig and the Clave people, and explain exactly what you're looking for and see what they say. They each will know their products better than anyone, and that will give you an idea on what is best for your application.
