I'd say go for the rope. It's the most versatile treasure tool. Also I think life in more important that ninjutsu espeacilly on someone like Roshi who scales with damage taken. As long as Roshi is on the field he's a thread.
The best tool for him is actually fan on an attacking lineup and gourd on a definsive lineup, but since you can't run both then just go with rope that is a midway.