Sorry, I didn't mean to imply that the 300mm was that much better, I was just trying to say how much bigger it is and still fits well in the Micro Trekker 200, which you can get for $50. You're making the right lens choice in my opinion, although you might want to consider the 70-200 f/4L which is about $550 now at B&H after the rebate. It's arguably the best bang for your buck L glass out there.
The only reason I got the 300mm f/4L first was because my son only had four football games left and I knew I needed a fast 300mm and couldn't afford the 300mm f/2.8L, otherwise one of the 70-200s would have been my first choice. A 300mm prime is a very specific lens for a very specific use, and I probably won't use it much until next summer. The 75-300mm or the 70-200mm will be a much better all around lens for you.