Thought it over, talked with a bunch of people. I am going with the Canon EF 70-200mm f/2.8L IS and a 1.4x extender which gets me pretty close to the zoom that I had with the 75-300 IS but much sharper and better quality. I can also get a 2.0x extender at some point and get all the way to 400mm. Flexibility!
Order placed -- should be here by Friday.