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.

I am assuming Rossi is your position 1?

If so go for the rope. Injury is the most important stat damage wise in the long run since it decreases crit damage (and increases crit damage dealt).

If roshi is not your position 1 then go for gourd or fan for the nin/def bonus.