SMS will work on both if both devices are logged into the same iCloud account in Messages' settings, if both devices are on iOS 8, and if you have it enabled in Settings > Messages > Text Message Forwarding. SMS may be a bit delayed on the iPad, since they must first go to the iPhone, then be forwarded to the iPad through Apple's servers.
iMessages will work if your phone number (or whatever address you are using) both shows up and is checked in Settings > Messages > Send & Receive. You'll want to make sure you have the same address (or phone number) checked as the Start New address in order to prevent confusion.
If you're iPhone's phone number is not showing up in Message settings on the iPad, sign out and back into iCloud at the top where it says Apple ID. On the iPad, that is.