Since everyone's pestering me about this, I'll get around to posting something.
I actually haven't tried Willow Moss and only tried Java Moss way back when I didn't realize I needed more than the standard light over my tanks to grow plants. As such I really can't give any definitive answers about either plant, especially in this application.
It's my understanding that most plants will do better in overly cold temps (as long as they don't freeze) than they will in overly warm temps.
If both plants are easy to get ahold of and fairly inexpensive, I'd try out both and see which works better. Otherwise I'd try which ever is easiest to get ahold of and/or the least expensive and see how it does first.