With the
JCW you pay for piece of mind with the warranty and ease if anything goes wrong as you can take your car to any MINI dealer to sort under warranty.
Non
JCW Tuners may give you more bang for your buck, or if your prefer more bhp for your £££ but then you have the worry if anything brakes on you if its covered under warranty, and whether the tuner will cover it, which may mean a visit to a tuner on the other side of the country.
You makes your bed and you sleep in it

I know which way I went and no regrets so far, but then again I haven't pulley'd it yet (major source or powere upgrade on an S)