The company I used to work for used SiteComp before I got there. By all accounts it was a good software, but they switched because you needed a special machine (some HP model, as I recall) to run it. You couldn't just buy it and stick it on your Apple or PC. I don't know anything about it personally, this is hearsay, but you should at least verify that you can put it on a regular machine.
I have used Terramodel and AutoCAD.
TMOD is adequate in its abilities, but extremely un-user friendly (lots of clicking and commands to do simple tasks).
ACAD is my favorite for ease of use and universal acceptance - I used to do work in CA, USA for clients in Hong Kong, and we never had to convert anything! But I have heard the modules (plug-ins to do specific tasks) are less user friendly than other softwares.
Our company also uses Microstation extensively. I have never used it, but those who use both say the modules are easier to use than ACAD.
I have never heard of Carlson Software, so I can't comment on it.
Generally, your students will absolutely have to use (or have software that interacts fluently with) AutoCAD in the civil field. It is, for better or worse (ongoing, endless debate here!) the industry standard.