Heroku & DigitalOcean etc, are just services which make deploying to "cloud" hosting easier. Heroku uses AWS's computing infrastructure to host your apps, but provides an environment to help deploy them easier:
Why do people use Heroku when AWS is present? What distinguishes Heroku from AWS?
Heroku vs...
Some of the important points to note about Heroku is you don't have direct access to your database, as well as relying on their infrastructure to hold up (E.G if something happens to their datacenters -- which has happened before -- you're at the mercy of Heroku & AWS's engineers).
There are two issues we have with them:
- You're obliged to use their AWS Postgres DB
- You've got to route through Heroku's domain
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Choice
The choice you really have is whether to use something like RackSpace (like us), or use a service such as Heroku or DigitalOcean. To me, there is not much difference, as Heroku / DO just provide an environment to push to VPS's running on their respective partner hosting providers.
With the ease of deployment (git push), you get the downside, which is your database, environment etc will not be directly configurable. I.E if you use Heroku, it's very difficult to get off it (transferring data etc).
By using a direct solution, such as RackSpace or AWS, you are in control of the environment, which means you get direct access to your database, dependent services & other aspects of the system. Of course, it comes with more maintenance, but you aren't locked into one system, as you are with Heroku
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System
In terms of the system, Heroku locks you into using their domain infrastructure:
The domain example.herokuapp.com will always remain active, even if
you’ve set up a custom domain. If you want users to use the custom
domain exclusively, your app should send HTTP status 301 Moved
Permanently to tell web browsers to use the custom domain. The Host
HTTP request header field will show which domain the user is trying to
access; send a redirect if that field is example.herokuapp.com.
This is coupled with the lock-in of using the likes of different dependencies & other aspects of the Heroku system. It's basically a store-front for Amazon's AWS infrastructure
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Recommendation
If you expect to have 5k users, I would highly recommend looking at
setting up your own environment, probably with RackSpace (as we
use them).
It's my opinion that Heroku is great for developers who just want to get an app up-and-running, but it's not very good with providing an environment you can control.
Some examples of this include your database - setting up on Rackspace etc means you get to define your database yourself. On Heroku, the db is kept in a third party datacenter, which you never see (try setting up PHPMyAdmin on Heroku)
Another example would be how you're going to scale the application. If you have memory leaks etc, you'll be far better positioned to have your own stack in place, rather than relying on someone else's. It allows you to define the specific details of how the app should operate - giving the ability to scale much easier
