I can't directly answer your question I have only used shared hosting, dedicated servers, and EC2.
However, I would say that EC2 is actually not that expensive compared to the others. I've been using it for my latest project and really like it. The cost is only about $75 a month and it behaves just like my dedicated servers (but it has more RAM!). That's coffee money.
Also, you will eventually leave the "development stage" and finding a place where you can easily (and without great expense) move to a larger VH will save you money in the long run, too.
$75/Month is way more than the $20/month slicehost is offering. They do offer a lot more memory, but for the time being, I just can't justify $75/month.
However, I would say that EC2 is actually not that expensive compared to the others. I've been using it for my latest project and really like it. The cost is only about $75 a month and it behaves just like my dedicated servers (but it has more RAM!). That's coffee money.
Also, you will eventually leave the "development stage" and finding a place where you can easily (and without great expense) move to a larger VH will save you money in the long run, too.