Well, you can share those 6GB with your iPad at no extra charge, or pay an extra monthly fee and have cellular access on your iPad. How much do you value the convenience? How much to you anticipate using the iPad away from wifi?
I have an unlimited data plan on my iPad for $30 a month. I also have 5GB on my iPhone. I chose this route because I use my iPad a lot on the go, and I didn't want to deal with the hotspot, I wanted the GPS, and I wanted the always on connection. My budget and my needs made this the best choice for me. YMMV.