The 511 has a better webcam. Nice if you use Skype. Also has USB 3
The 3 series has a "non-reflective" screen. Can be an advantage, if you want to use it outdoors, but some people don't like their non-reflectives, so worth actually check them in store, to ensure you're happy with it.
Other than above, main difference in RAM and HDD.
The 511 is better, overall. Whether it's £150 better ???
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