The only real differences between the Net+ and the CCNA is that the latter is much more specialized. They both center around networking, so you're going to learn many of the same things. Cable specs, differences in equipment (router vs switch vs hub), OSI model, IP addresses, a basic on DNS, a basic on common network services, etc. The Cisco material will demand a much better understanding of all this, and will also expect you to know Cisco's IOS, a management console built into most of their routers and switches.
On the job market, you're likely to get a bit farther with just a CCNA rather than a Network+. Also, the Cisco cert will play a much bigger part only if you're applying for jobs in an large-to-Enterprise size business. A CCNA is overkill for a local mom-and-pop computer shop, or geek squad.