I was faced with similar decision, 35i with Dinan tune or 35is. But my first concern was with the warranty (which I don't know if the BMW dealership would honor on a mod car). I have decided on the 35is (and if I still like the car after 4 years, will consider the Dinan upgrade). In the meantime would have the small additional power of the 35is over the 35i.
Not having a manual transmission is not a show-stopper for me. My cars are daily drivers through some city traffic and I would prefer the DCT.