Ok, I'm going to try to break down the different models the best I understand them:
306 and 306T
These models appear exactly the same to me and are the same price. They have a day and night setting so each outlet is in control of one of the settings. This is for use with incubation periods that require different temperatures between day and night. However, if used with both set up identically, it should turn on both outlets at the same time, and then be able to turn on both heaters, if one is plugged into each outlet. It is rated for a max of 10A, which comes out to be 1200W at 120V.
308 and 308S
These models are also exactly the same except the 308T comes with the longer probe. I believe that the longer probe can be used if you don't want to fully submerge the smaller probe. These units are true temperature controllers in that they have heating and cooling capabilities, and will keep the temperature steady, no matter if it's getting too warm or to cool. The probes on these units are removable and replaceable with a simple headphone jack style plug. These are also rated for 10A, which is 1200W at 120V.
310T
This unit operates the same as the 308, but has 6 period settings that allow you to program it to keep certain temperatures during certain periods. For instance, if you're brewing and need to have the temp at one setting for a few hours, then lower for a few days, and then higher for a few more days. This controller can handle up to six of those periods. It is not really useful for our aquarium use, unless you want to use it to induce breeding.
So, that's the breakdown, as I understand it. Lol
Btw, these units are actually rated for amperage, not watts. They are rated for 10A and power (watts) is calculated by amperage times voltage. So, at 240V (which this controller can handle) it can output 2400W.