I currently have an MP7 that I use as a backup but going against the theme of this thread I use AEG's as my primary go to setup at the moment, but I always have it in my kit as its reliable and I don't have to rely on batteries as long as I have gas.
My thinking behind this rational is that if my primary AEG goes down for batteries I can grab the MP7 and game on as most sites sell gas Whereas batteries and charging facilities are a different matter.
But that's my theory and it might be skewed.
Just to note I normally take at least 2 x AEG's to a game and MP7 GBB. All depending on the site and if my mates are going as well but I have been known to have 4 AEG's especially if I have travelled for a site. But that's the luxury of having multiple rifs to choose from.