Dave Bland explains how you can use DEADLOCK_PRIORITY to control which process gets rolled back:
Before getting into how to set the DEADLOCK_PRIORITY, let’s quickly go over what the options are. There are two ways to set the priority of a process. The first option is to use one of the keywords, LOW, NORMAL or HIGH. The second approach is to use a numeric value between -10 and 10. The lowest value will be chosen as the victim. For example, LOW will be the victim of the other process is HIGH and -1 will be the victim if the other process is greater than -1.
As I recall, index operations (like rebuilds) are automatically set to a low priority.