A friend of mine and I have been working on a startup for the last 8 months. It has been tough as we have been working full time for a company and hacking like crazy once we got home, or on weekends.
We chose to use Heroku because of the dead simple workflow that would allow us to spend time developing features instead of maintaining the infrastructure. This may sound like a cliché, but boy was it a good decision. The platform is rock solid, deployments are a bless and the add-on system is just one of the best time savers you can get.
We know OpsWorks (or earlier Scalarium) from our full-time job, and we know how cool it is. But we also know that it requires a lot of time and additional effort to keep things running.
This may pay off if you run a company which can afford it, but you should think about it twice if you have limited resources (and other, more important things to do!).
Even now, after we have launched, we still see no reason why we should move from Heroku to OpsWorks (or another platform). We have made some optimizations and clever (to us) decisions to avoid running into scalability problems. Maybe we will think about migrating once we get a ton more clients and even more traffic. It is cheaper to just add some dynos than spending time on server administration.
In the end, it's all about making the right decision, so don't hurry and if you think Heroku is too expensive for your business, maybe you are not making enough revenue from your customers. On the other hand, if you have the money and time, than go for it.