Something to consider is that DO doesn't offer internal network IP's. So if you have something like a cache farm, you have to access them via external IP's. Linode is supposed to internal IP's, although some manual tickering is required. I'm not sure about EC2, but I'll assume they do.
On a cost per hour basis, DO is the cheapest. Linode next. And Amazon most expensive.
Also, DO is only located in NY, while the others offer central and west cost locations.