I've used 12d for over 10 years, and in my brief time with Civil3D i could see it was far more user friendly, spoke to other BIM software much more smoothly, and offered everything inside the AEC package, with no additional licenses. One look on the 12d forums (which you need to buy a license just to access) will show you they seem almost allergic to modernising their user interface, it's famously clunky, their support is outsourced to a private company, and their user manual is 8,000 pages and counting (it's funny because they put the button for opening this manual right next to the "Confirm" button when doing alignments, so you often misclick and end up having pdf viewer open this massive document). Their visualisation package (which until model 14 required a paid license) is also astoundingly bad considering how long they've had to develop it.
Despite its lack of polish, 12d i found was the far more powerful software for creating surfaces. Civil3D constantly froze or crashed on me especially when using lidar information. I can't speak to its design functionality because i didn't spend much time designing in it.
In Australia and New Zealand 12d pretty much has a monopoly on the civil engineering market, however this is changing rapidly. More and more recruiters are starting to add Civil3D as a requirement for new roles. 12d has hastily been playing catchup the last few years introducing things and trying to keep up with the rapidly emerging BIM environment (which it is still mostly terrible at). 12d's renderer is OpenGL which should tell you all you need to know about how good the visualisation is (Infraworks etc. use gaming engines for theirs so you can imagine the difference when someone puts a VR headset on. If i was in a meeting room and a client asked me to quickly change the alignment of a road to see how it interfaces with a neighbouring development, for example, i would get nervous as doing this in 12d, while it has improved, is still clunky. In Civil3D you can just pick point and move them, and everything else updates dynamically.
I've been told another problem with our reluctancy to switch over to Civil3D entirely is its in built stormwater functionality. We use pit & pipe dynamic drainage using the ILSAX method typically in Australia which is built into 12d. I think Civil3D uses the SCS runoff method, which 12d also has (if you have the dynamic drainage license). It also has TUFLOW (flood modelling), HECRAS functionality, and can read Drains (another australian stormwater software) models. At this time i'm unsure if Civil3D has any addons which can satisfy the need of the local designer here. This is huge as local counsels require more and more the submission of stormwater models in these formats on top of the hydrology/hydraulics calculation sheets.
And don't even mention drafting in 12d. A painstaking process which they've tried to implement within the software that nobody uses .... so ultimately we still require two people & two bits of software (usually) to finish a product. People say "use a .dwt" from 12d to CAD, but there shouldn't even be a middle man. I understand Synergy fixes a lot of cad woes people have in 12d, but this is another package you need to pay for. They are masters for squeezing every cent out of the consumer.
I'm sure there's other things i could dump on here but for now that's my 2 cents.
In summary, i use 12d because it's required of my role. But i wouldn't be surprised for a second if the market (which it has done in other countries like the UK and UAE) switches overnight to Civil3D.
