Neither!!! (or tape measure if you must)
Weight fluctuates incredibly from person to person, from day to day, from week to week. Throw your scales away - it doesn't give you the best picture! The best things to measure are things that directly determine weight - that you have full control over (hormones, water, food - all have more immediate control over your weight than you would like).
These "best things" to measure include 1)number of exercises, 2)length of exercise, 3)type of exercise, 4)heart rate, steps taken, etc AND caloric intake. To help you out, look into getting a heart rate monitor (Polar makes great ones). These often come equiped with software that tracks all of this for you and lets you export to Excel.
Just an opinion...but it's one I've used in working with morbidly obese people to get great results.
Stephan