for weight loss, i always say the treadmill...i LOVE my treadmill and it burns most calories as it's a full cardio workout
if you want to avoid adding muscle to your legs then avoid the bike as it's a cardio workout BUT the main muscle usage is the legs.
rowing machine is a good one for keeping upper body in good shape, it's killer on the shoulders and biceps, but that's a great thing!lol
cross trainer is one more for the arms so if you've got bingo wings this will be the best for you
overall they are ALL cardio so good for weight loss, so i say go with whichever one you prefer!
do they not have instructors that have written you up a workout program?