Most fat burning occurs AFTER exercise not during, due to the calories required for recovery and altered hormone levels caused by exercise of appropriate intensity, so do NOT worry about what rep range or heart rate burns more fat.
More important is the total number of reps per exercise per session. Most shoot for 24-30 total reps. This can be achieved by performing 2 set of 12 reps, 3 sets of 8-10 reps, 4 sets of 6 reps, 5 sets of 5 reps, 10 sets of 3 reps, etc. Most people find some form of cycling different rep and set ranges over time promotes the most long term improvement in both strength and body composition. Typical cycle periods are every other workout, or every 2 weeks, every 6 weeks, etc.
Or you can use different reps ranges for different exercises. For example, use lower reps (3-8) for compound movements and higher reps (10-15) for isolation movements.
Or you can use pyramid sets, where each consecutive set you use more wieght and do less reps; or or wave loading, where every set you use a different weight and rep range, typically using more and less reps on every other set.