My trips in the last 18 months on NX and Megabus have leaned toward NX for better journey quality. They have announcements saying where you are etc, they have the obligatory safety announcement too, and I like their buses more. Also feels like you get more leg room on the NX. NX also run more buses in general and have a more "integrated" network.
Looks like maybe Megabus have new buses though based on this thread, so I'll consider them for my next journey. The downside with Megabus is they start and end their journeys at absurd times; if you're not in the start/end city as part of your journey, you're stranded for hours. I.e. Birmingham to Glasgow leaves at 2am. The return journeys get you into Birmingham at 3am!
(though there does now seem to be a daytime journey too).
Megabus Gold seems new and shiny, but I'm not keen on leather seats.
Megabus have better customer service; NX fobs you off and seem very corporate, but Megabus have been willing to offer refunds without my even asking for them for things I've brought up.
For a quid its better to have it than not to have it in my opinion
I think that's what they're counting on.