Now back on topic.
I had the Dinan stage 1 and stage 2 tunes on my M3 before getting the ESS Supercharger. I really liked both Dinan tunes and they definitely provided more power. I never had an error code from either of their tunes. They drove smooth and had some added power when I wanted it. I was wanting more power so that is why I went with the ESS Supercharger.
Now that being said, ESS did dyno my M3 before putting the supercharger on and for fun they put their own tune on my M3 and dyno'd. They made about 15hp-18hp more than the Dinan tune. Keep in mind that this was only for fun as they were installing the Supercharger on my M3 later that same day.
In summary, Dinan has always been a little more conservative with their products, but their power increase numbers are accurate. Keep in mind that Dinan has to warranty all their products so that factors into the cost and into the level of power increase that they feel comfortable with. Of the two tunes Dinan vs. ESS, the ESS will make more power for sure. Their support has been second to none for me, even with my 1M.
I hope this helps you out with your decision.