I would use the 125 just because the length gives them some room to chase and swim. Either would be fine, though.
The rule of thumb is usually 1 discus per 10 gallons, with a minimum of a 55 gallon tank to grow out a group of 6. Are you planning to set this up as a bare bottom tank or a decorated/display tank? There are a couple routes you can go depending on your goals.
Most discus keepers raise out young fish in bare bottom tanks with lots of food and water changes. It's a pain, but it gets the maximum growth out of the fish and ensures that they develop to the full potential that their genetics allow. Bare bottom tanks are easier to keep clean which is the main reason for it.
If you're not up to large and/or frequent water changes, you could always start with larger fish. This would allow you to cut back on the water changes and feeding without worrying too much about stunting the fishes' growth.