Flying a mission to intentionally study a moon is a bit different than accidentally contaminating it while getting no useful data. Titan isn't a likely candidate for contamination anyway and if we ever do find life there it is likely to be distinguishable from Earth life. Enceladus, on the other hand, is one of the likeliest places for Earth-like life in our solar system and much more succeptible to contamination.

Since the orbiter is out of fuel and will eventually crash into something, Saturn makes the most sense.