What were your reasons for buying the iPad? If they were the same reasons for getting a smartphone, then yeah, you might be better off with just an iPhone.
Or, you may just not know the full potential of your iPad. Are you an artist? Do you like apps like Paper or Procreate?
Do you have a photo share setup that you'd like to browse through DS Photo+?
Or maybe you want to take notes in school/work on OneNote or Endnote. Perhaps you'd care to play a pretty random puzzle game called Machinarium, or "Manchurian," as it's incorrectly called in our house.
Teamviewer or remote desktop apps work much better on an iPad than an iPhone, and the same goes with reading apps such as Goodreader, which is excellent for PDFs.
In other words, the iPad is definitely an entirely different device than your iPhone, regardless of all the striking similarities. If you're just using it for email, web browsing and facetime though, then you may want to consider selling the iPad and cashing back in on some major buckage $$$.
I use my Ipad more than my iPhone because my eyes aren't as good as they used to, and the batteries last longer on the iPad... also, I don't limit my iPad usage only at home. I take it everywhere I go. The only time I use my iPhone more is when I upgrade to a new iPhone, and/or if a new jailbreak comes out. Other than that... the iPad is a lot easier on my eyes, and I love the extra real estate on HD apps.