Just to quickly clarrify - the Sony NEX system is NOT an SLR camera. The NEX system fits into the category called Mirrorless camera or CCS (compact camera system) Basically you can still get everything you want from an SLR and a lot of great little features like art filters. BUT you DONT get a mirror inside the camera body - you can still change lenses but in an SLR, there is a mirror.
Benefits of a mirror = you get an OPTICAL viewfinder THROUGH the lens. You can see through the lens (cameras like the Fuji Xpro1 uses a seperate view finder.) Also Cameras that have mirrors MOST OF THE TIME (And I say most in terms of not all the time) Most times SLR cameras can focus faster and take a picture soon after.
Disadvantages of using a mirror = Larger body - Live view is often left wanting in most cameras that use a mirror.
Benefits of having no mirror - Companies like Olympus and Panasonic have perfected the face detection and constract detection AF in their system being almost as good as an SLR - they are the current leaders in AF in the mirrorless system, but still not as fast as some serious SLR cameras out there. Another benefit is the small size of the cameras and the smaller size of the lenses. You get ARt filters and other little gimmiky features you may like - most cameras now come with a touch screen AND an electronic viewfinder. The touch screen can let you do point focus faster, just by touching the area you want to focus on.
Disadvantages of having no mirror - as I said, the AF is not as snappy as some other SLRs. Accessing manual functions is often difficult or limited. Buying a zoom lens often just kills the reason for buying a small camera body.
So in my opinion - Id just get a SLR if you are serious about getting a proper lens system. If its just a once off purchase for a holiday or something, getting a twin lens kit for a NEX camera is a better choice.
Hope that helps
1) Olympus OM1 [Zuiko Auto-S 50mm f/1.8]
2) Pentax MZ-60 [Sigma 28-90 & 100-300]
3) Canon 7D [EF-S 15-85 & 70-200mm f/4 IS & 50mm f1.4]
4) Leica M [50mm Summicron Pre-aspherical - Silver]http://www.poetproductions.net