Hi
konradh The reason I prefer to master sometime later is simple really. The main reason is that after a long session of mixing your ears are just not in the right space to be making important mastering decisions. I can guarantee if you apply say overall EQ to your mix at the time of mixing and then do it a week later the overall EQ setting you end up with will be different to the EQ setting at the time of the mix.
If you read the books 'Behind The Glass' I and II many great engineers in there say when they mastered an album either at the time of the mix or the next day it was a big mistake. It was due to pressures from record companies and the like.
When you send a mix to a mastering engineer they are fresh in the morning usually and they have never heard your mix before so they tend to approach it from a very fresh perspective. If you are mastering yourself you can sort of do the same thing by leaving it a week before mastering. When you hear it again it all tends to sound a little fresher and you tend to approach things like EQ, compression and limiting from a different angle. I do anyway.
Another good reason too is it is great to live with a mix for a week. I listen to it in the car, at home while having a dinner party, on various speakers etc.
(not my main monitors so much though during that week) All sorts of things pop up when you do this and you inevitably change a few things about the mix. And if you don't then obviously you have nailed the mix which is great because it also means no further work.
Gentle two buss compression over a mix can be good and it is possible to setup that compressor to just change things very slightly. I sometimes do that if I am aiming for a very loud master at the end of the project. It can help towards that a bit. It is like applying that very thin coat of compression paint first.
Another great reason is, it is very handy having a full mix with no processing applied in your backups. It can still be 24 bit etc. Then if you don't like how you have mastered something you can also go back and change it. I like listening to a mastered track(s) too for a while after. You sometimes pick up some places where you may have overdone a process here and there. If you have that unprocessed mix available it is so easy to just recall everything and readjust.