I have Bluefin on mine - very happy with it, 22k miles of trouble free motoring so far - rough figures in my signature under 'VCDS plot'. Not the cheapest option, but customer service is fantastic. Can't comment on Revo, but I'm sure someone will be along to give you their view.
I've had GIAC on my petrol S3 but have used Bluefin on 5 different applications, 3 of which were 2.0tdi, no complaints at all with excellent customer service, Superchips will offer a 15% discount to you as a forum member.
Can't comment on Revo as stuck with Bluefin due to personal experience but they are very reputable if a lttle expensive IMO.