I have an LLC Photography and Design business, it is a side business to my main source of income (which has nothing to do with photography). I can only comment on a sole entity ownership. I obviously call all my own shots, I make all the decisions, etc, so I die by the sword. I am considered a "Professional Photographer", according to the IRS, because I am booked for events, sell images, design posters, collect and pay taxes, etc, etc. I have accumulated over $10,000 in equipment. I got to a point where I thought this hobby needs some payback, so away I went. I can control how much work I take on to fit my schedule. That's my luxury. I get hired to cover sporting events, I dont do what I know I can't handle. I am not at the point where photography can be a full time occupation. I dont know if I ever really want it to be. I have it way too good in my full time job, with benefits, to give that away and become a starving photographer. I also have to think of my wife and 2 kids, so to me, i have a healthy balance. This, is just some of MY benefits of being on my own.
I cannot comment on a partner setup, I think that would be something to hear from existing partner set up successes and failures. That should have an impact on your thought process.
As far as a business plan, I have one, but it is pretty pathetic. Im kind of a "wing it" type of guy. It is highly recommended though, buy business types that any new business have a plan, and if banks and lending institutions are required for set up, it better be iron clad.
I'm not going to tell you what you should do. There are lots of people in this world who take phenominal photographs and their business sense is terrible. Having a successful business is only so much about taking a good picture, its about having good business sense as well. Very few people understand this!