This topic has been beaten to death a thousand times before but I'll give you the highlights:
JB4 is only worth it if you have warranty and would like to minimize the risk of losing it
Flash tune, depending on the tuner, is the more refined product but it leaves traces and makes it easier for BMW to deny a warranty claim
I wouldn't touch RaceChip with a stick because of their aggressive marketing strategy, their
man-buns and the fact that they do hundreds of cars and don't seem very specialized on a specific engine. They do offer warranty though but be careful about the Terms and Conditions.
Up until recently a Flash Tune could be done through OBD port. This was patched and now you're forced to physically remove the ECU. It's do-able, but I wouldn't trust anyone that hasn't already laid hands on the S55, so be sure the shop knows their shit and its not all talk.
Also, take a moment and search for the Crank-Hub issue. If it spins, there's a good chance that (with either piggyback or flash tune) BMW will void your warranty and that leaves you with an expensive reparation bill. Fixing the crank-hub is also expensive though and you'll be putting a lot of confidence in the shop's skills because they will need to dismantle and re-install a shit load of components. There's a few detailed videos of the procedure on YouTube, make up your mind there.