You will choose the fee that you think is right for the urgency of your transactions. The difference will be that the amount of space available will multiply by 2-4x, depending on how many signatures you're using in each tx. So, for instance, if you're sending from 100's of different addresses in one transaction, you'll max out the new structure nicely. And because of the extra space, the expected fees will drop by 2-4x also. This is a more complex way of simply making the space bigger regardless of whether the data is just the tx part or it's signature. It's a compromise between straight blocksize increases and leaving things as they are.