I would NOT second the Yamaha recommendation, as I've NEVER been impressed with their ability to create a transparent view into music. All of their HT products as well as their Integrated receivers have a veiled sound to me, which creates a separation and involvement with what I'm trying to hear.
Of the two choices you're interested in feedback on, I'd select the Marantz over the Onkyo.
Of the two choices you're interested in feedback on, I'd select the Marantz over the Onkyo.