Calculating calories burned

I use a fitbit and have it synced to MFP.

I've got a fitbit zip. The only exercise I do is walking, so I don't need the fancy Heart Rate Monitor devices. I've had it since March 2015, I wear it every second of the day, except for the shower and bed.

I rely on it so much, I would never ever be without it now.

I started out using MFP's activities. I know have a Garmin 920xt which also counts steps to add to my daily activity.

I've been successful using both.
I originally lost my weight with MFP's numbers (then got pregnant).
Here is a good blog to help decide
http://www.myfitnesspal.com/blog/Azdak/view/estimating-calories-activity-databases-198041
http://www.myfitnesspal.com/blog/Azdak/view/exercise-calories-sometimes-the-cardio-machines-are-more-accurate-404739