What speakers are you going to use with either receiver? I like the sound of both brands and the user friendliness of Onkyo's but the Marantz line has better multichannel power than Onkyo. Onkyo employs a protection circuit that limits output in multichannel mode after 2 seconds or so. The real world effect of this is a subject of debate and Onkyo is not the only brand who does this[Yamaha also] but since Marantz, H/K, NAD,Elite don't need to do this I ask why would you choose to. Poor heat sinks? Weak power supplies? Anyway, listen to both and try to become as familiar with both as much as possible before you decide.
I've decided on B&W speakers, either the 603s or CM-2s. (Any thoughts on these?)