For me its both the song AND production. I suppose it depends on your individual goal. Personally speaking, I didn’t invest my hard earned money in professional hardware and software just to create good demos. If you use the right tools (and there are many great freeware plugins for that) and research how to master properly, you can still have the mix you painstakingly created along with the added “glue” and loudness. If you’re deviating that much from your mix when mastering then you’re doing something wrong. Please don’t take this the wrong way. I am not saying everyone should master their material and you’re wrong if you don’t. I am just defending the point that throughout modern recording, there has always been a post mix process to unify material to a standard as well as improve its sonic qualities. The process shouldn’t be dismissed entirely and has its place whether you master one song or a collection of songs.
Recently I’ve been using this with the Steiny brickwall limiter in Wavelab 8 and have been very pleased with the results. It’s easy to overdo but if you understand how it works you can get some great results. I’ve also placed this on my masterbuss in the project and “printed” to my DA-3000 with great results as well.