If you haven't done 3D before, SketchUp is probably the best choice to start learning because it's free and you can get a grip on it literally in a matter of hours. You won't be able to create complex organic models in it, but SketchUp was built for making architecture so it can be incredibly useful for building environments. Its Warehouse has tons of free models (including whole cities!) for every occasion so in many cases you'll be able to use already made models as references.
If you want make characters and complex environments as well, Blender will be the best choice because it is also free and multiplatform and I have to say that Blender became a very powerful combine of almost everything so you'll be able to model, sculpt, texture, generate foliage and do many other things. The problem is that Blender has a very steep learning curve and can be very counterintuitive in places, especially for those who didn't came to Blender from other 3D software, but once you get a grip on it, Blender will become quite easy to handle and incredibly useful for all sorts of tasks. The main catch is – if you want Blender to work effectively, don't be afraid to tune it up for your own preferences as Blender is very flexible and can be customized to fit you like a glove. The second thing is - just like Firefox, there are all sorts of addons for Blender that can greatly help you in your workflow, but they can be tricky to find because you'll have to know exactly what you are looking for.
Right now I'm working on my own comic which is being made almost entirely in Blender and unfortunately I have to agree with others that making stuff in 3D takes longer than drawing the same stuff in 2D, but 3D has many advantages over 2D such as much greater flexibility and re-usability of assets, but it requires at least some experience.
Overall, 3D can be a tremendous helper for complex scenes and that's why many professional concept artists use 3D drafts as basis for their drawings. I've spent several last years learning specifically how to create complex environments in Blender and I can tell that there are tons of nuances you'll have to get through to be effective, but I firmly believe that with right pipelines and techniques almost anything will be possible.