iMovie is supposed to be great, so I don't really think you need an upgrade. I use Premiere Pro and am delighted with it, It took a while to get used to it but once you know what you're doing, you can do pretty much anything
Sony Vegas is and always will be the best editing software i have ever used no matter what people tells me, If this is better or that's better i am comfortable with using Sony Vegas pro. so i would seriously recommend this!
Vegas is ridiculously expensive but I use it and it's pretty worth it especially when you become adept at using it. However for something more affordable you can use camstasia to edit/record and there is also lightworks which is a free editing software.
definitely i recommend using camtasia studio 8 i use it to make all my videos and wouldnt use anything else, its also cheap if you want to buy it but if you download the trial and it expires theres ways to get around it, ive just recently posted a video on it if you were to use the software. definitely try it out before purchasing anything else