If someone is in a pay per post campaign then he will try to post the maximum number of posts counted in that payment period,hence creating spam.
....so that, one won't be able to login to their alts and start posting, right?
But in a fixed payment campaign, the person only have to post minimum number of posts required for the payment, as more Posts will not pay him, so he will not spam.
Yes, it will be easier to maintain a minimum of 25 posts through all of their alt accounts, within the timeframe.
Every type has its own merits and demerits and its better if you leave this decision for the campaign owners. Each have their own way of initiating a campaign, thinking as to how well they can expose their niche. Also budget is another factor for them. So its not you who should decide after all.