I much prefer Pocket's abilities to handle multiple media types.

I like how Pocket is a first-class citizen on all of the platforms I use, not just iOS. I know Instapaper has an Android app, but it sucks.

I prefer the design of Pocket over Instapaper. I'm not a designer, so I can't comment on which is actually "better", only which I prefer.

I do prefer Instapaper's business model. I would like a chance to pay for Pocket.

Finally, I am not a huge fan of Marco Arment, and would rather support Pocket than him.