you don't want a C Corp. it would pay federal and maybe state income taxes on its income and then you would be taxed on any money you would take out, An S-corp avoids the federal taxes at the corporate level but there may be state income taxes and there may also be annual franchise taxes.
I would say the choice is between a partnership or an LLC that is taxed as a partnership. only the broker could tell you if an LLC is permissible. should it loose more than is put in, would your limited liability protect you and the other member from having to make up the loss. the brokerage may want personal guarantees if it is an LLC . a partnership/llc offers more flexibility in allocating income should there be a reason to allocate income to r loss differently. by the way, so both of you are protected there should be a written agreement about how the entity operates and splits income and expenses. sometimes things blow up and then you end up in court when each saying they agreed to something else. that makes the judge the referee.
