The most popular CAD tools nowadays are:
LDD is made by LEGO itself, though it's not really maintained or updated anymore nowadays (it does sometimes get a sudden Brick update). It has brick-snapping and collision detection which makes it really easy to use and learn, but it can sometimes be a pain to work with (especially with more advanced models and Technic models) due to that same system. Cause it's a proprietary system there's not a whole lot of tools that use the LDD file format (but there are a few).
LDCad is based on the LDraw parts library which is open-source and thus has many, many more parts than LDD. LDCad has become the go-to LDraw CAD nowadays (there are many more such a MLCad, LeoCad, BrickSmith etc. etc.) because it's modern, fast and has brick-snapping (no collision detection though). The opinions about the UI are mixed; some hate it, some don't mind at all. LDCad does have a steeper learning curve than LDD or stud.io but it's also more advanced. The file-format is open source and thus there are a bazillion of tools out there that support the LDraw format.
Stud.io is the new kid on the block. It's made by Bricklink and is a hybrid between LDD and LDCad (or any other LDraw CAD). It has both brick-snapping and collision detection and it can connect with the Bricklink database to show which parts exists in which colors and what the price is. The file-format is a bit weird though. It is proprietary, but based on LDraw. The .io files itself can thus be easily exported to LDraw.
If you want to build Technic models, I'd recommend you use either LDCad or stud.io with my personal preference going to LDCad (disclaimer: because I'm using LDCad and all other LDraw tools for years and years now :P).
In the past, there was a great tool called SR3D-Builder. It was a bit like LDCad, but its main focus was (Technic) simulation. You could move gears, open doors, extend/retract pneumatics etc. etc. It was a great, great tool! Sadly the author passed away a couple of years ago ?