Sportline HRM vs Polar FT4 Lower calorie burn with Sportline

I have the Sportline HRM which I purchased from Walmart some time back for around $50. It does have a chest strap. I recently changed the batteries in the watch and chest strap. The watch is programmed with ht., wt., age, sex and I can choose to set it for low, medium, high activity as well as fat burning, aerobic or fitness zone. I purchased the Polar FT4 and used it for the first time yesterday. The reason I bought the Polar is because I didn't feel the Sportline was giving me accurate calorie burn counts. For the 30 DS level 2 I was only burning 135 calories. Last night I wore both the Sportline and the Polar for 30 Day Shred Level 3 and the Sportline gave me a calorie burn of 165 and the Polar gave me a burn of 185. Now I'm not sure which one to use, and which is more accurate. I read great reviews of the Polar so that's why I decided to buy that one. The Polar has my HR limits set at 122-160 and the Sportline had it set for 122-140. Does that make a difference? Also, I should mention that I have not lost any wt. in over 3 weeks. Not sure if that's due to using the 30 DS and I am likely to be gaining muscle or if I'm not eating enough of my exercise calories back because I have been using the Sportline??? Any thoughts would be greatly appreciated.