It... depends. There are certainly some 100% brown rice syrup beers made out on this forum (I think most of them are under Dkershner's page, since we don't have a gluten free recipe list yet). There are taste and texture differences involved when doing different ones. What you should really do is compare yourself. Find what both the taste and mouthfeel differences are, and then decide (and contribute) which situations you feel that 100% BRS is better than mostly sorghum, and vice versa.
There are studies out there that show sorghum as being closest to barley for brewing uses, and rice would be similar to... well.. rice. So you're getting some differences. So your options are: To not use sorghum at all, based on hearsay. Or to test it out first, and based on your experiences, not use sorghum at all, mitigate any effects with other fermentables, hops, flavorings and other changes.