I can give you a professional's view as the architect and CAD manager I am
Have no experience with 3DHA so cannot help you with that.
If you're determined to let go of 3DHA salvage everything you can first. Try to export your documents, drawings, models, details and construction parts libraries. Anything that you might want to reuse. It's usually better to export data from within the program that created it than later trying to import it on another program that claims to understand it. To this end your best bet is .DWG format. Although it is a closed proprietary format it's usually a better bridge for data than say .DXF which is open and standard.
As yksyks first told you SketchUp
is the way to go. It's very well put together, very intuitive. Responds very satisfactory to professional requirements but is equally accessible to curious DIY user types. It has already gathered a strong user base which makes help and content and add-ons readily available. And because it has Google to back it up you can be sure that it won't disappear from one day to the other or be gobbled up by Autodesk as so many good ones have. SketchUp is genuinely good and I think many small practices will adopt it.
Forget about AutoCAD. It saddens me to say this because I'm sort of an ACAD nerd and proud of it
. I'm 37y.o. and have 20 of experience with it. I know nearly every aspect of the program and have earned at least as much money from this self taught knowledge than from my architectural work. I say AutoCAD is on its way out. AutoDesk's REVIT
will take its place on big architectural/construction firms. Autodesk won't say it and will milk the ACAD cow until it dries up. ACAD had its time but now the paradigm is changing from the virtual drawing board or the virtual 3D look-alike model. Nowadays the focus is on assembling a virtual building/database/model that not only looks like or helps build the real thing but is built like, functions like and helps the real thing function well beyond its construction. REVIT is what I'll learn next.