At least to my ears, the AVR300 is much better than the Marantz sonically, especially for 2 channel. In fact, I've found the Arcam to completely smoke any HT receiver I've auditioned, even those at twice the price. You would have to buy much $$$ more expensive separates to equal or surpass its performance IMO.
The pre/pro section is outstanding, much better than the Marantz - in fact it's as good as many high end separate processors. The amps are excellent and should power most speakers just fine. I found the Arcam more spacious, open and dynamic than the Marantz, with more clarity and resolution - this is key for music listening. It may be due to the preamp section, the power supplies, the spearation, the amps - or more likely - a little but of all of them.
The set up is very easy for music/HT as well. The processing has many cool, user-friedly features. The tuner is just OK, and that is true of virtually every HT receiver I've heard.
If you care as much or more about about music as you do about HT, the Arcam is a no brainer. Nothing comes close at its price, new or used. The Marantzes and Denons (of which I've owned two each) don't really stack up.