what are your fitness goals?
Spinning classes are cardio workouts.
Weight training builds muscle mass.
that being said, muscle mass helps with cardio - like climbing hills, but good cardio gives you the endurance to handle the hills.
staying in the high repetition lighter weight range won't build bulk.
building bulks is VERY hard and time consuming, and takes many many calories. most of us don't even eat enough to sustain our body weight let alone gain extra.
so do both! (with good form of course) and dont' forget to stretch (with good form).