I will give you my experience with the downloads vs DVD formats.
I have both, I have the STS series in DVD, and I have abs and stretch routines as downloads. At some point I will order the STS downloads and gift my DVD set to someone ready to get started. I have also pre ordered the new low impact series as download.
I believe they are exactly the same, but I prefer the download version.
I have little use for extra things on my shelves, and with the downloads you don't have to worry about that.
I keep them on my computer which is in my workout room, and I keep them on external backup. I can also make them portable and load them on my laptop to travel and I won't have to keep track of DVD cases.
A friend of mine loves having the physical cases to show off her library, I just don't like to use space that way.