If I didn't need dual USB output why I would I buy the newer as the older model is shorter by 1.7cm or 10% and lighter by 30g or 10%. See specifications comparison and size comparison
Older model also has higher capacity battery as said by Unboxing the New Xiaomi Power Bank 210000mAh | ChargerLAB article
Although the new Xiaomi Power Bank 2 and Xiaomi Power Bank 2 both use the lithium battery manufactured by Tianjin Lishen, the old model uses the M74 4.4V 38.5Wh high-density lithium battery while the new Xiaomi Power Bank uses conventional voltage lithium N22 4.2V 37Wh. The new one also features dual USB-A ports and a bigger PCB, resulting in a 17mm length increase when compared to the old model.
It is now slightly more expensive but only by a few dollars as I guess stock gets depeleted.
3 incoming!