Don't forget the lock in. If you've been using Instapaper for 2 years, you might have a huge archive and a couple of good friends that share interesting stuff. You'd loose all of that by switching to Read It Later.
One thing that I sorely miss in Instapaper though is the ability to mark text fragments and save them as quotes (like in iBooks for example), and add notes to it. I'm reading all these texts, and I can't mark quotes or note my thoughts. I hate it. Does Read It Later offer such a feature?
You can export your stuff from Instapaper as CSV. Read it Later lets you export your stuff as HTML. If you wanted to switch you could probably script the migration in a few minutes.
One thing that I sorely miss in Instapaper though is the ability to mark text fragments and save them as quotes (like in iBooks for example), and add notes to it. I'm reading all these texts, and I can't mark quotes or note my thoughts. I hate it. Does Read It Later offer such a feature?