To expand on my previous post Building Cloud Connectors: What is a Feeder? let's briefly discuss the general use case for each.
Step-based connectors are generally used to update or add data to records that already exist in Eloqua. The true power of a step-based connector is the use of Program Builder to perform operations (such as Lead Scoring) on contacts before they are sent to your connector step. This gives the marketer a robust system to target and process data, and get the most out of the use of your service.
If your goal is to build an integration with a system that contains contact data that is not already in Eloqua, then a Feeder is the way to go. A Feeder will allow marketers to reach out to your system and pull new leads (as contact records and custom data objects) into a contact group in Eloqua. Once the new contacts are in Eloqua, their group can be used to "feed" a Program Builder program.
In order to offer the most flexibility, and drive adoption of your service, offering both types of connectors may be the best strategy.
As far as coding effort, both types are developed using the same basic principles.
For more information, see Building On The Eloqua Platform - A Resource Guide