What a difference a slab makes.

I was just checking prices on 2007 Little Rock commems. A NGC MS70 or PF70 will run you between $50 and $80. The same coin in a PCGS slab will cost between $250 and $280. What is the difference? Both are reputable grading services. If I was to buy the NGC graded coins and send them to PCGS to crossover, which they should, the value of my coins would increase by $200 each. I see coins graded by PCGS almost always command a higher price. I currently have my coins graded by NGC however if this trend doesn't cease I will need to change graders. I believe in a certain amount of loyalty but not at these price differences. It would seem that we need a new grading scale which would include lets say a 70.1 for coins that are better than a regular 70.