I have mine set at 50 for both radios (in a 3-level single family home). However, achieving 300 Mbps on 2.4 is unrealistic and only happens on occasions for short periods (usually, my closest clients are at 216-270 Mbps). So I just set it to 20 MHz, and I get the same throughput at a consistent 130 Mbps link. Setting it at 20 MHz is also a neighbor-friendly thing to do.
For 300 Mbps on 2.4, I think you have to be at Ch 1, 6, 11, WPA2/AES, WMM enabled.
For 5 GHz, I find that it is consistently at 300 Mbps even for remote clients with only two bars (out of 5) signal strength. That is why I think I am going to get some more dual band adapters. It is extremely fast on 5 GHz.