Alot of it is about your time and the money you have to spend. I do both. I make quite a bit of terrain for stores when I work at them as well as for my self, but I also buy a fair amount if its good stuff and decently priced. Yes armor cast stuff was great but was priced a bit high ( dont be fooled I own loads of it ) I do mostely 40K terrain.
That Ausy company has fantastic stuff and the price for the quality of what you get I dont think can be beat. I bought 6 of those adobe style building for my Orks, the went together like a snap, I used wood glue to give them some strength ans well as mounted them to MDF board, they are great and dirt cheap for what they are. I have plans for quite a large gothic ruins set and at least 3 or so of those drop pods. I highly recommend them and if you in the US the Warsotre has them for order.
Like I said if you have loads of time and no money then scratch build the stuff. If you have some coin you can buy some great stuff at model rail road shops and other hobby stores to inhance your scratch building. No time but disposable income. Buy the stuff and call it done.