I can say from using both brands that you cant go wrong with either. I had the 2515's for sometime, when I decided to upgrade the 9515's was just more then I wished to spend. I then switched to the DC XL's which are very comparable to the 9515s based on motor size and specs. I was not dissapointed with the switch. Its not fair to say one is better the the other. BUT with you asking which is better beween the 2500 and level 4? The level 4 should win hands down. bigger coils, and larger motor. Now with a comparison between the 3500 and level 4 they should be the same. I will post a side by side pic later of the 3515 and the level 4 15 later when I can since I have both sitting here in the garage.