Johnathan,
Thanks for the introduction! First off, I have to make the disclaimer that I have not used Beanstalk with any technology other than Java so my comments may not be accurate for PHP, Python, etc...
I am not familiar with dotcloud but from what I can tell there is a lot more flexibility with it which also means a lot more configuration. ElasticBeanstalk is really great for a vanilla app server. You can do some customization, much more than I am familiar with, but it cannot be customize as much as some may want. Simple example is that I know some people prefer a Java app server other than Tomcat and that is not available at this time.
Pricing is complicated to compare because it looks like dotCloud is a flat fee whereas Amazon charges for data transfer and other things.
One thing I can say about ElasticBeanstalk from experience is that in my experience it works very well for growing and shrinking as necessary and for removing instances that are having troubles connecting. dotCloud lists an autoHeal feature but I am not sure what that is exactly. Best thing to do would be to get started with beanstalk on the free tier, which I think you can do, and do a free trial and see which fits you best.