When given the choice between onboard, Xonar U5¹ and Prodigy Cube² under $100 one would probably go for the highest quality product available. A few years ago I would have chosen the Prodigy Cube, because it has designer components and an impressive professional looking product page. I bit later I would have gone with professional recording interfaces, but unless you can actually evaluate the objective audio performance of a device, a discussion will be more subjective based on taste and preference.
- Try onboard first. You are already willing to pay for it and it can be your fallback if the external soundcard drops out.
- I recommended to read a review about the Behringer UCA202 (~$30) in: External USB audiocard with 3.5 or 6.35 jack output. I feel a bit ashamed for dropping the name in every second answer of mine, but that is as much as I know or have experience with.
Asus support staff was unable to answer a question about demanding low impedance headphones regarding their high-end soundcards. I have little to no confidence recommending their products if "supports 600Ω" is what they can say about it.
I was interested in buying a Dr.DAC from Audiotrak until I read that the Alps potentiometer usually crackles and that this can be avoided in high quality designs.