I'm thinking of getting a sensor to work with my cell to track rides. However my phone is on a per mb data plan. Am I going to burn huge data using a sensor that pairs with my phone? Would I be better off just getting a fitbit?

Thanks!

No, sensors usually connect to phone using Bluetooth (on some phones may use ANT+ instead), so they have no relation to your data plan.
Fitbit has nothing to do with bike sensors, so your last question is kind of strange...

There are apps that will track your GPS location so it can be plotted on a map. But if you have any specific sensors in mind for specific data, perhaps you could add that info?

You don't pay for GPS data. You can turn data off on your phone when you ride.

I don't use sensors on my bike. I just let the phone use GPS. It's a little less accurate, especially as I ride. I think after I upload the ride, it gets recalculated better. I could add a speed sensor, but I just don't bother.

I use ridewithgps. There are other apps, and I think Strava is the most popular.