If you've ever owned both..and in my experience, the bill, after a brake/rotor job, for starters. Besides, the iS models are more likely to be driven harder.

OP...the TC is a common and inherent failure-prone component on the 4.6iS. In my case, it happened with two separate 4.6's between 60k-80k miles. Albeit, i replaced the later as a preventive measure when i first suspected a trany issue. A neglected/failing TC will eventually cause damage, even to a rebuilt/new trany.

I became aware of this issue when i joined this forum, by member JMH...his 4.6iS was an inspiration to many of my mods. You can read about his X5 here, if interested: JMH's X5 4.6iS Home