my 2 cents
I have been editing movies for the past 3 years and I used istudio, vegas, gopro studio and premiere pro. I would say, gopro studio is just for real amateurs, no different than movie maker. istudio does the job but lacks lot of features. Sony vegas and premiere pro is a close combat. vegas I feel more flexible, but if you are going to use any other adobe products on your edits such as after effects etc then premier would be better choice. on the flip side, vegas requires more ram to work with but GPU support is quite efficient if you have compatible graphics card.