The Marantz and the Denon are very similar in power output, just based on maximum power into 4 ohms I doubt anyone could tell them apart by listening. They are within 1 dB of each other according to their specs.
What is more important is how they handle various speaker loads - low impedance or high impedance phase angle can demand a lot of current from an amplifier and that is where not all amplifiers are equal in performance. Of course, the mfg's never specify performance as regards to low impedances or difficult loads. Some amps do specify maximum current output which can give some idea of how they handle real world loads. Just based on their market positions, I'm guessing that neither the Denon nor the Marantz could handle a "difficult" speaker very well except at modest output levels.
Some speakers are VERY easy to drive and with such speakers either of these amps would be fine. The Devore Gibbon is one such. Polk speakers are usually quite easy to drive. People say that Klipsch horn speakers and JBLs are easy to drive but I've never liked the way they sounded. And "easy to drive" is NOT the same as high efficiency, though efficiency often does come into the picture.
When manufacturers rate output power of amplifiers they drive a signal into a plain old resistor. All a resistor does is take electrical power from the amplifier and turn it into heat. But we don't use amplifiers driving resistors to heat our rooms - we use amplifiers driving SPEAKERS to reproduce music. And a speaker is **NOT** a resistor. A speaker doesn't just take the power from the amplifier at any frequency and turn it into heat. A speaker reacts differently to power from the amplifier at different audio frequencies, and at some frequencies it will sort of "fight back" against the amplifier, becoming more difficult to drive at certain frequencies and so on. It's quite complex, and there are many very fine speakers that are quite difficult to drive. Read some speaker reviews in Stereophile in terms of impedance analysis and you'll get an idea of the differences in speakers in terms of "driveability."