Highlander Limited Platinum - which is the trim with all the fancy premium leather and such that the RX350 has - costs $57k. Coincidentally, that's also the starting price for the RX350. So there's not really any extra $ involved. You can spec up the RX to get some fancier seats and a bunch of tech stuff that I guess isn't offered on the Highlander, but there's no need to do that. So apples to apples you're not really paying much extra for that Lexus badge.
The interior of the RX is a little fancier / more elegant than the Highlander Platinum. Probably just a matter of taste, though.
I of course would say don't buy either - they're both crossovers. Stop buying crossovers.