So to answer your question, BOTH will fit in the Antec case you listed and YES the power will be good for your GTX680.
The HX850 has slightly better power efficiency at 80 Plus Silver vs Bronze and then will be able to keep the inside of your case looking MUCH Cleaner and organized as you wont have all unused cables bundled up at the top sitting there collecting dust and taking up space.
Is it worth the price difference? Personally i'd go with the modular one but both will serve your purposes just fine