It depends on the kind of label you wish to print. For waterproof text and simple single color graphics (black usually) thermal printing is king. If you instead need a full color photo or graphic design and text label that can look as good as a hollywood movie DVD, then inkjet printing gets the nod. The only weakness inkjet disc printing has is it is not waterproof or smudge-proof like a thermal print. (But DVDs shouldn’t be allowed to get wet or be handled with greasy fingers ANYWAY. )
Taiyo Yuden offers a fairly waterproof inkjet printable disc called the Watershield; Verbatim has something similar called the Aqua Ace.