Which zoom - SAL70300G or SAL70400G

I'm fairly new to DSLRs having purchased my first (Sony A77) just a few months ago. This fall, I plan on shooting a lot of soccer games. I'm having trouble deciding on a lens - SAL70300G or SAL70400G.

How much better is the 70-400 than the 70-300? Worth double the price? Do I really need the 300-400 range for soccer? Part of me wants to say go for the SAL70200G instead of the 70-400 with it's constant f2.8, especially with the high shutter speeds I'll need for sports photography, but I know I'll end up having to blow up / crop photos since the 200 just won't reach a full field (probably about 80-100 yards to the furthest part of the field from where I'll be standing).

Any help before I spend $1000-$2000 would be greatly appreciated. If you have other lenses that are of equal quality, please recommend!

Thanks in advance!