What's the difference between blocking a phone number with Google Voice compared to blocking with iOS 7?
For the recipient, there's really no difference since their phone won't receive calls, voicemails, or text messages. But, for the caller, it's a slightly different experience between the two platforms.
Calls blocked with Google Voice will receive a message that the number is no longer in service.
Calls blocked with iOS 7 ring for the caller (not the recipient) and then the voicemail (or text message) just disappears (update: actually, the voicemail goes into a Blocked Calls folder on the recipient's iPhone). To the sender, it appears they were delivered.