Can't speak to Anthem long-term reliability, but my previous Denon 4308 crapped its digital audio board just a few months outside of the warranty period, and Denon's response was "it'll cost you around $550 to fix, IF we have the part still." So while the sample size is too small to give meaningful data about failure rate, it is IMO sufficient to show that Denon is not a firm one can trust for reasonable and competent after-sales support in the event of a problem. A customer who bought a $2500 MSRP box can and should expect better after-sales support. I won't be buying from them again.
That said, the major selling point of Anthem is IMO that ARC is the best of the room correction systems available on affordable current-production gear. But unless you need/want the streaming capability, there's no reason to go with the 500 over the 300. The power difference is de minimis in real life, and the 300 is significantly cheaper. (And if you do need the streaming capability, I'd say get a 300 and an AppleTV...)