There is no doubt that it should be Nianfei.
(1) From the point of view:
Before Yongzheng acceded to the throne, Nian Fei was a serious side Fujin, second only to Ulana La, while Xi Fei was just a princess at that time and didn't even have the official identity of Jade Butterfly. If you really like her, don't you even give her a formal birthright?
After Yongzheng acceded to the throne, the young princess was crowned the imperial concubine at the beginning, and at the end of her life (at this time, her mother's family had completely lost power and influence), she became the only imperial concubine in Yongzheng; As the birth mother of the May 4th Movement, Fei has no difference and no special treatment. In the eighth year of Yongzheng, Jin Guifei was obviously paving the way for Li Hong, and at that time, she was favored by Qian Yong.
(2) In terms of fertility:
Because both of them have children, they must be capable, so the frequency problem can reflect the goodwill.
Nianfei, from the fifty-fourth year of Kangxi to the first year of Yongzheng, gave birth to three sons and one daughter in nine years; However, Xi Fei only gave birth to a child once, and considering that Xi Fei has always been healthy and lived to be over 80 years old. What this represents, I believe it is already obvious.