SSD for OS and software, HDD for video files because yes, its much more cost effective at this point. There's some comments here about the longevity of SSD's under constant use, that they are more susceptible to fail earlier then HDD, however alot of the literature coming out on that fact is showing it isn't necessarily true. But the cost for a reputable 2TB SSD drive on Amazon right now with a quick search shows it close to $700. A 4 TB WD Purple at 5400 rpm (lower RPM is better for energy consumption btw), is around $144 and you can prob find them on sale for a little cheaper if you're patient and feel like searching. I'm sure after a few years once the cost drops it'll be much easier for me to move away from HDD to an all SSD system.
I'm currently trying to figure that location questionactually.. I bought a SFF Optiplex meaning its very small and i'm told will be pretty hard to add much inside, although i've also read elsewhere that it is possible to remove the CD/DVD drive (if you have one) and purchase a caddy to hold a 2.5" SSD in the same spot where the optical drive should be. The PC hasn't arrived yet but i have ordered some parts to see if this method will work on mine. I've also read that its possible to literally velcro or double sided sticky tape the SSD into the tower as well. fenderman is an admin on here and i've found him to be a great source of information with regards to PC capabilities. There are a few others that seem to really know what they are talking about so i'd browse around and take some notes. Thats pretty much what i've done.