Been a couple of times to the Grand Canyon but have always driven there whilst touring. I would highly recommend visiting to anyone but can"t help with the Las Vegas companies.
I guess there are 4 choices of how to visit. Helicopter, Light Aircraft, Coach, Hire car.
If you pick either of the road versions I would say doing it from Vegas is a bad option unless you plan on staying overnight. Its just too far to travel and the time you get at the Canyon is too short. If you do plan to stay over then this would be my recommended option.
If you fly then then the time spent at/in the Canyon would be my deciding factor rather than the price. Once you get there you will realise what I mean when I say the views are priceless.
Havn"t looked into it but if I was going from Vegas I would take a helicopter into the Canyon and a raft down it.
Again, don"t take my word for it, but there always seem to be LOADS of offers on the Helicoter flights once you are in Vegas.