You can get more information on a Blu-ray Disc. For example a SD DVD is usually on a 4.7 gig disk. Now you cannot use all that on the media we put on it, as there has to be room for the navigation. The max we ever use is 4.5, and generally we try to keep it 4.3 or 4.4 gigs. Now a Blu-Ray that we use holds about 25 gigs of data. So you can do the same program which might come to 21 gigs, so you know the quality is better because there is more information from the original source material(Shot from the camera). In the old days of VHS tapes, the weakest link was the VHS home copy that was sold. Usually pros shot on pro camera gear, which kept the quality as good as possible for the time. Thus when DVD's came along the old master tapes were converted to DVD, and looked better than the VHS.
To me the idea of buying a Blu-Ray simply as a back-up for your existing DVD's is a sound idea!
Greg