Choice can be difficult between these two. I have not used FIIO E10K but on quick google search it seems its a nice DAC. But somehow I have a feeling that DX will be better sounding than FIIO Dac. Since E10k is a USB Dac it has its own advantages. You can use it with your laptop as well and sound quality difference between xonar DG/DX and FIIO E10K might not be apparent since you dont have any hifi headphones.
So you can go with any of them DG/DX or E10K, you wont be disappointed. Xonar DG will cost half the price of E10K I guess, If you think you laptop sounds fine enough then go for DG/Used DX or if you think you want to get the best out of both systems then you can consider E10K.