Quote: My understanding of the bigger sensors (FF) is that the pixels are larger, larger pixels allow in more light thus allowing faster shots in lower light.
The difficulty with this line of thinking is the assumption (always unstated) that in good light, 4/3 or APS-c or even point and shoots aren't getting enough light. As long as your smaller sensor gets enough light, it's fine. There is a narrow gap, where APS_c is unacceptable, and FF is acceptable, before FF become unacceptable...but it's a really small window. For some it's a critical window. Think of it as a scale 15 inches long, broken one inch sections. After the APS_c image becomes unacceptable, at say pick a number, like 8 on the scale, between 8 and 9.2, there's a red zone, where you get acceptable images with FF and not with APS_c. Now the area may be between 7 and 8.2 or between 12 and 13.2, that's pretty much personal preference. But that's the difference. Now you have to decide. Will I pay twice as much for that difference. The answer for some pros and advanced amateurs is going to be, yes it's essential. If you're one of those ones, then FF is essential to you. But for most of us, we'll stay lower budget and be happier that way.
The other advantages of APS-c far outweigh that little 1.2 inch red zone on the 15 inch scale that determines when an FF is better than APS-c, or Two inches for 4/3, or 4 inches for a point and shoot. By the time you've arrived at the point and shoot you've lost between a quarter and a third of your functionality. But APS-c to FF just isn't that much.
If you are a wedding pro and 50% of your work is in that red zone after APS_c but before FF cuts out, then absolutely twice as much money for an FF is a good deal, but in actual practice, a D4s or low light specialty camera is a better deal. A standard low cost FF dslr is still a compromise, and now your'er getting up into 645z pricing for your FF (with the D4s), and the choice to go MF becomes a lot easier.
The choice isn't D600 or 645z, it's D4s or 645z...and the 645z gives you a heck of a lot more resolution.