This is a very interesting discussion and I am going to be the first to say synthetic and mummy shape. I have used a Domex brand synthetic mummy shape for the last decade and it has served me well although to be fair the coldest that I would have used it would have been -6 and I was using a silk liner as I was fairly exposed under a hootchie half shelter. I would suggest you consider budget first, comfort and fit (always get in a bag before buying it) second and then the possibility of getting wet in your bag (never a recommendation) thirdly.
If you can afford it then go down as the others have recommended. As to standard oblong shape or mummy shape that really is entirely personal preference but if there is even the remotest of possibilities that your bag could get wet while in use then go synthetic (you will get just as wet as in a down bag but you will be heaps warmer).
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