Are you going to stick with zooms or use primes? How important is a viewfinder?
Both those cameras are good - the G3 (micro four thirds) has a better selection of native lenses if you wanted to use primes (the Panasonic 14mm (£120 ebay) and 20mm (£210) and Olympus 45mm 1.8 (£220) are particularly good as is the more expensive Pan/Leica 25mm).
The Nex lenses are bigger and the Nex has no viewfinder (an add on one is £200). Overall I would say the Nex has better image quality and has the edge in low light/high iso situations.
I have the older Nex 5 and an Olympus E-PL3 (micro four thirds camera) and both are great. Overall I prefer the Nex and use it more often, but I have a lot of fun with the E-PL3 and 20mm lens which makes a very compact package. To get a similarly small combo with the Nex I need to use the 16mm lens, which I find too wide for general use and IQ not as good as the 20mm Panasonic. The G3 is bigger than the Olympus Pens (E-PL3, EPM1) though.
[Crop factor on the micro four thirds cameras is x2 by the way vs 1.5 for the Nex and Eos 400d]
For the Nex, good lenses are 16mm (for compact size, for landscapes (it's best at f5.6 and f8) and because you can add the wide angle adapter (£90) to the front for another fun option; - plus the 18-55 kit zoom is decent for a kit zoom, the 55-210 zoom I like a lot (it's the lens I use the most) and the 50mm 1.8 is said to be pretty decent too although I don't own this.
Main benefits of the G3 - faster AF than the Nex, built in evf, SLR style body and controls, better choice of native prime lenses.
Main benefits of the Nex 5n - probably better overall IQ, better low light/high iso performance, more dynamic range, focus peaking for manual focus.
The Sony has a few nifty features like in camera auto panoramas, hand held twilight mode and and in camera auto hdr mode - all work in jpeg only as they take 3 pictures (or more for panorama) and combine them in camera.
