[eykanal]

They're two completely different products. With instapaper, you visit a page, click a bookmarklet, and the page is saved to your instapaper account. You can then go back later and read the article. It's basically a huge "read later" list. There are a couple very cool features (read text-only, export to pdf|epub|other, iPad/iPod apps, ...) which really make it very good at doing what it does.

RTM is a general purpose to-do list. Theoretically, you could add a to-do with the URL and have it function as a read later list. However, the software is designed to be a to-do list, and the added functionality of instapaper will not be there, and if you ask the developers to implement, say, a preview feature for a URL they'll probably look at you funny and ignore you.