An UPDATE:
I did get both to compare. Decided agains the P&M one quite quickly - the colour was a bit wishy washy and just didn't like it. The Whistles one however is lovely.
But... now I have another dilemma! I thought maybe navy is a good colour and would look good against brown and tan jackets. So looking again on P&M website I found this bag:
www.peonyandmoore.com/product_info.php?products_id=225
Which is also really lovely! Nice shape and size for me, not too big not too small and lovely colour.
So now I am torn between navy P&M and the grey Whistles one!