You are right that it'd be two primes vs. the one 18-55mm kit. But, you have to take into consideration that those two lenses (35 f/1.8 and 50 f/1.8) both blow away the kit lens in terms of quality. With the kit lens, you are sacrificing image quality for the ability to have more focal length choices in one lens. That's the trade-off.
It all depends what and where you're going to shoot. If you shoot outside with good light you may not have many problems. But if you like shooting indoors with limited light, the 1.8 aperture is going to come in handy.
The 50mm f/1.8 is a great lens at a great price ($140 new, around $100 used).