Differences in capability between cross-cut, plunge, and traditional circular saws?
You're right that you can do plunge cuts with all of them. The plunge cut saw is designed for more precise control of depth and position of such cuts. It's really more of an industry specialty tool. 

It's not clear to me what the differences are between the cross cut and circular saws. I would have a look at them in person and see what features they each have. It appears that one of them may have an exhaust port designed to connect a vacuum. It may therefore be more expensive due to features that you don't need.

Nearly all professional carpenters as well as homeowners in my area used basic circular saws. Try to find one with smooth, quiet bearings and a good reputation for quality. If you need to cut cement board siding, purchase one with sealed bearings. If conversation is important, look for one with the automatic brake, which stops the blade fairly quickly after a cut.