the 300c looks the part but build quality is poor and going on from what people have said (including my dad) they are plastic feeling and cheap. the 350z is the more powerful and looks good, but then the 300c and 350z appeal to two different groups of cars in terms of looks.
the 350 uses the VQ35DE (depending on year) 3.5 v6 engine and puts out 280bhp. the 350z has a great engine sound, but if you get the 300c r/t 5.7 v8 hemi that will too!
i'd go for the 350z because i prefer sports cars and the reliablilty and quality will be much better than the 300c.