The main difference between Grace Ice & Water Shield and Grace Ultra is that while the adhesive in Grace Ice & Water Shield is based on SBS rubber, the adhesive in Grace Ultra has a butyl rubber base. There are other differences as well. Grace Ultra may be left exposed to the sun for up to 60 days, and it is slightly thinner at 30 mil than Grace Ice & Water Shield, which is 40 mil thick.
Stated simply, butyl rubber offers exceptional stability at high temperatures. In some roofing applications, such as those in the desert southwest, roof temperatures can get high enough that SBS based adhesives can flow under the extreme heat. Butyl rubber based membranes remain stable for long periods of time even at extreme roof top temperatures.