Remember the Milk also has an Android application of its own (available in the market). Haven't used Astrid, so don't know which interface is better.
Especially on the website, I like RTM's ability to make numerous lists. On the mobile app, you can view items by list as well, but wherever possible I like to do the heavy lifting on the big screen.
This allows me to just look at the web dev list, the home repair list, the office maintenance list, the sales lead list, etc. Or if I just want to despair, I can also look at a view of tasks from ALL lists.
Caveat, I'm a RTM Pro member, which is necessary to sync RTM with apps like Google Calendar (at one time I was using this to work it around to syncing with Outlook in a Rube Goldberg fashion). I'm NOT sure if you need the paid membership to sync from their website to their own mobile app. If so, it's only $25 a year. The app itself was free.