Sometimes the slightly more expensive option is the cheapest option in the long run. I have seen used, in very good condition to mint examples go in the low $400s. And it is hands a down a very much better lens than any 70-75-300mm zoom. Plus, if or when, the OP has no more use for it, it will have a better resale value. Had OP done this at the start he would not be buying the same lens twice!
OP wants to shoot baseball perhaps some wildlife. Obviously he wants better than he has gotten so far. As to the size or "heft" of one of the 150-600mm super zooms, it does depend on how much you want the best photos possible. If the size or "heft" is too much for you, you probably need to find another hobby as this is what it takes.
I have let rather smaller stature high school girls use my 1DX with the SIgma 150-600mm Sport (with monopod) on it to shoot football for yearbook. It does all depend on how bad you want the shot.
EB
EOS 1DX and 1D Mk IV and less lenses then before!
