Diet soda's sweeteners can make your body crave more sugar/sweets after you drink it, so although you're saving the calories, you may want to have other sweets even more later on. See if you notice an impact, but regardless switching to diet over regular is better, there's no doubting that.
Here's a scale to 440lbs, that's cheap, but to go above 480 you need to spend at least $150 or so.
DIGIWEIGH DW-68G HEAVY-DUTY PERSONAL BATHROOM SCALE