Which lense on the Sony NEX 3 or NEX 5?

I'd advise against a NEX in first instance. You'll get normal DSLR image quality but terrible ergonomics, a tiny selection of lenses, and the size advantage of the body is negated by the size of the lenses again. Also, the future of the system is rather dubious, i'd rather not invest in lenses for a camera system that may not be supported anymore in ten years.