Liz's answer makes sense. For live music photography, one more stop can make the difference. The f/f lens will save you some money and some weight. The f/2.8L weighs a ton. I have the non-IS version. Working with that sucker all day does a number on my shoulder and arm. But I do like it. I've thought about getting the f/4 lens myself, just to have a lighter option. In the end, you have to decide if that extra stop is worth the weight and cash. Think about this - how will you feel when you're trying to shoot at f/4 and you need one more stop and your lens doesn't have it. Is that going to happen a lot? If so, the f/2.8 lens is probably the right way to go. If not, buy the f/4 version and save a little money and protect your arm.