Comparatively speaking, the big three long distance companies are pretty much the same in terms of rates. For instance, if you call Sprint and tell them your long-distance needs, they should be able to get you a rate. If you call AT&T right after and tell them what price Sprint gave you, chances are AT&T has a similar plan. It's the other factors that will probably influence your decision on which long-distance carrier you should go with (ie. airline miles, quality, cost of owning their phone card, customer service, etc.).
If you are the type who makes a lot of long distance phone calls from home, allow me to make a suggestion: Sign up for a cell phone that has free long distance or has nationwide rates. You will save yourself lotsa money because you will not be paying them by the minute. For instance, I use T-Mobile as my cell phone provider... but I also use them as my long distance carrier. If I need to make a LD call, I just use my cell phone. I get free nights and weekends so I usually call everyone I know in the country on the weekends all day long and pay the same price every month.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!