So really the only difference in them is whether or not they have an enclosure, bed heater, and how large the build volume is? No different software or anything?

Correct.

You have some questions here that you should search and learn about, they are common for 3D printing.

All single extruder. There is at least one user who has modded theirs to dual extrusion. Search in the hacks and mods section.

Does the enclosure help in any way? What is the point of it other than looks?

Also, if I design something on autodesk inventor, how to I go about printing it?

My final question is are all of them a one filament at a time? How many extruders are there?

This has been a huge help.

SD3 has a 8x8x8" build volume. It comes with a bed heater, but no stock enclosure. Many people purchase this and make thier own enclosure with simple cut plexiglass/Lexan. This is better for those who will want to tinker, but dont need to do so immediately.

SD4 has 8x8x8" build volume and comes with bed heater and enclosure. Best option for people who do not want to upgrade the machine on their own.