Freelance Photographer Seeks to Upgrade Equipment, Gain Notoriety in the Photography Biz

I've been doing freelance portrait photography for quite a while, and am curious about boosting my camera equipment so I can get more serious. I've been shooting with a Nikon D3100, with two lenses (the kit 18-55, and 55-200mm). Not only is it very frustrating having to switch lenses when I need to zoom at all, I feel like I constantly need to clean dust out the body when I'm finished for having it exposed for the couple seconds it takes to switch. I definitely want to invest in the 18-200mm lens, but I'm curious about switching bodies.

I'm really comfortable with Nikon, so I might want to stay with them. What's the pros of Canon over Nikon? I'm thinking about going to the D5100, because it has in camera HDR, external mic input for video (which I've been doing a lot more  of) and higher ISO levels with less noise.

What do you guys think? Is it worth the trouble to upgrade from the d3100 to 5100, or from Nikon to Canon? I see this becoming my career because I'm going to school for graphic design and photography, but I don't have a lot of cash on hand. I'd want to upgrade by selling my equipment and purchasing new equipment for not much more.

I also want to get my hands on a softbox, for indoor shoots. I've been relying on natural light most of the time. Is a soft box a good starting point for indoor photography?

Thanks for reading my first Verge post!