John Crowe , Mar 28, 2010; 09:44 p.m.
Generally I prefer cropping from images made with a 1.4x over using a 2x. However some situations, like my kids' soccer games, lend themselves to using the 2x to get even more reach while maintaining the flexibility of a zoom. For wildlife I would still prefer using a 1.4x until I could afford a long telephoto prime, like the 300/4 L or 400/5.6 L. There are times when the longest lens you have is still not long enough and there is no point in always using a zoom at it's longest setting. This is when it is time to buy a telephoto prime.