I'd gladly sacrifice that element of "defensive specialty" for a more well-rounded player who is good at everything. I think that's what Toronto is aiming for with JV at the 5. There's no doubt in my mind that JV will be a better all-around NBA centre than Biyombo (who I agree has elite shot-blocking/rebounding potential).
In more ways than one Biyombo reminds me of a terrific centre of not too long ago...Dikembe Mutombo. All defense mostly. That maybe his ceiling but Mutombo had a good grounding in the fundamentals at Georgetown.
I don't think he will reach that potential but I believe it could happen if everything falls right.
I see JV having a lower ceiling but also being much more likely to achieve the minor star level.