My focus is on making a lean, cross browser, bookmarking site with a focus on short-term storage. If you don't opt-out of the feature you can get a reminder the day before any links expire in case you forget to read one.
This app grew out of the habit I had of emailing articles to myself several times a week so I could go back and read them after work, or at lunch, or whenever. I realized there had to be a better way. Looked at a few of the other sites around at that time, didn't really like any of them, so I wrote my own.
One of the big reasons I wrote my own was that I'm addicted to interfaces like Hacker News and Reddit. Simple, concise, and almost exclusively text only. I modeled the link-list on that idea. The one twist being, thanks to some nifty JS code, that the longer a link hangs out on your list the more faded it becomes. To me this makes it a much quicker and easier interface to get in, find what I need, and get out. I carried the same idea through to the bookmarklet in that it doesn't redirect you to the MarkitForLater site when adding a link. Instead it adds it in the background and notifies you once it's done, this way you can continue reading, or move on with minimal interruption.
In summary, I think MarkitForLater provides a minimal impact on the way you browse, while still providing a useful service.
I think Instapaper is one hell of a cool application and has definitely earned it's place in the market. I won't tell you which is better for you, but I know which one I use ;)