Balls are the thing that produces semen and are overall an important organ that keeps men's hormonal balance in check - basically what makes men men. By leaving only the shaft you're getting the equivalent of castration, which means one would have reduced testosterone and no sexual drive (as proven many times by troons who go through vaginoplasty). You're losing sexual satisfaction with either choice, so I'd rather choose the one that still allows me to create life and have a family consisting of my own kids.
Edit: Actually, you have three choices here. "For one you shall keep, and one you shall lose." Men have two balls. If you choose "balls" that means you lose one, but keep the other one.