Trust me, I get what you're saying. Everyone falls in love with the violin and never the viola (some are even asking WTF is a viola). And I love Toni's velvety, smooth voice, and the vocal trills that define the Toni Braxton sound. Even with all that, I still think Tamar has a very, very, very slight edge. It's not necessarily that Tamar has the higher range, but it's her wider range (all else equal) which affords her alil more flexibility.
All in all, if I walk into a piano bar, and either of these two are singing, I'm going to be there for awhile (as long as Tamar isn't singing songs from her album).