I have done that trade a few months ago and sold my white 5711 to fund a 5320 which I loved at first sight!
The 5711 didn't get wrist-time anymore so it was an easy decision for me: If you don't wear it - sell it. Very happy with the upgrade to the 5320
Yes, it will be a nightmare to get the 5711 back and IMO it's more versatile than the 5320. If you have doubts and if you still adore and enjoy wearing your blue 5711 then it might not be a good idea to sell it. The 5320 will still be around in a few months so better take your time and make sure you really like it.
Good luck with your decision!
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