For a little over $500- the 70-200 f4L- hands down the best tele zoom Canon makes that costs less than $650.
I wouldn't even buy the 75-300 IS lens over the 100-300 EF. Image quality will be the same between these two (75-300 and 100-300), but the handling and AF performance of the 100-300, which uses the ring USM instead of the 75-300's micro USM, is head and shoulders above the 75-300. neither of these lenses are really sharp above 200mm though.
After looking at all the available lenses in this range up to 300mm, if I were not going to get the 70-200 f4L I would go with the Sigma 100-300 f4 HSM, which is like ring USM. The Sigma will be better than either the 75-300 or 100-300 Canon lenses.