For those of you lucky enough to grow and taste both, is there an appreciable difference between the two in terms of flavor and productivity? Or if grown in the exact same climate and blind-taste-tested, would anybody be able to tell the difference?

I think in my opinion the RDB would win by a large margin in a blind taste test agains't VDB.

because it's sweeter?

For me rdb is way more productive and a much faster grower.  VDB is better tasting but for me too slow growing.  I think the better question would be to compare rdb with negronne.

So how do they compare?

Negronne, tastes good but has tough skin. It's not as vigorous as Ronde de Bordeaux. 
RdB ripens earlier than negronne, has delicate sweet skin, and has the perfect fig flavor. 

I received a RdB Earlier this year and it had lost the head in transport, the side shoots haven't broken yet and it's been a bout a month but the stem is still green underneath, is it sensitive to stress, I am thinking it will start to grow soon but still it is really nerve racking waiting.

i had two RdB cuttings. one is growing nicely. the other one finally kicked it yesterday. i think the root is still good. i'll be leave it in the pot in hope that something will come up sooner or later.

i heard so many good things about RdB. can't wait till next year. i don't know what's going on, but all my cuttings are putting on figs. it's rather strange to me since none of my older trees put on the figs on first year. maybe it's this crazy weather.

Thanks to all for their insights, particularly Bass and Ed.

Rafed, please tell me a bit more about why you prefer RdB over VdB by such a large margin.  I've only tasted VdB and think it's great, but I guess I haven't had too many rare fig varieties for comparison.