Why would I choose Channels as opposed to Structures
From the official docs:

Channels are used for streams of similar content
Structures are good for times when you need to store multiple similar entries, and they need to be sorted into a specific order. They can also be hierarchical.

As per your question there's no performance overhead involved, but as a general rule it's usually a good idea to keep things as simple as possible.
Channels are bit simpler than Structures and if the content is neither hierarchical nor multi-level, there's no reason to use the latter. A blog is a good example of an appropriate use case for a Channel.