The main difference that I am pointing out here is that with Form and Particular, you can use OBJ files as particle emitters only, so any OBJ surfaces will be made out of particles, not solid fills.
With Element and Stardust you can also use OBJ files (and sequences) as particles (including displaying a single particle), which means you can render solid 3D objects like you would in any 3D software.
In my humble opinion, if you want the best return on investment, I would recommend Stardust. It has most of the features in the Trapcode Suite and Element 3D in one single integrated package. The main downside with Stardust is sparse documentation.
Kalleheikki Kannisto
Senior Graphic Designer