The reason is... it's a "bird in hand" vs "in the bush" arguement. You can rip the files from the DVD+R DL and burn them to -DL or whatever you want in the future.
Trust me, having the discs done is paramount compared to the issue of what they'll go on. In fact... when I was doing my project DL discs weren't even available, but I was working on a DL project because I didn't want to sacrifice quality asn knew that DL would be possible eventually.