you know I actually disagree a bit and think it's totally the other way around if anything.
with the singular exception of He Wasn't Man Enough, which felt very crossover, Toni has always stuck pretty rigidly to R&B. she's less versatile and her lane has always seemed more niche to me. her least R&B record is Pulse, which is a bit generic pop in places and came during the wilderness years when her commercial appeal had ended and suddenly nobody was interested. but on the whole she's really stuck to R&B and maybe if anything that both hastened her commercial demise in the '00s and gave her renewed credibility as a legacy act in the '10s.
whereas Mary has been way more all over the place. she came back in '07 and got a hit with Just Fine as the most straight-up pop song of her career, and half of that album (Growing Pains) was not R&B at all. then she flirted heavily with electropop beats on My Life II and with house music on The London Sessions - I think she saw the decline of R&B coming in the early-mid 00s and after The Breakthrough (more a neo-soul record anyway for the most part) jumped ship, which was probably quite savvy and good for her survival and relevance. but now, unlike Toni, she's not really delivering for the R&B queens who would be her core fanbase at this point in her career, and her last few albums flew very under the radar perhaps for that reason.
swings and roundabouts!