At the *worst* it'll be a push. If the material is only available in SD on iTunes, that'll (most likely) be the same as DVD quality. If it's available in HD on iTunes, then that'll trump the DVDs. But I've usually found that if something isn't available on blu-ray then it's likely not available in HD digitally.
So... buy whichever one is cheaper, unless you either A) specifically want the physical media, or B) specifically want cloud access/backup of the material. In those cases, get whichever one makes the most sense to you.