If I understand the question correctly, there is currently a partnership, and the issue is whether to organize as an LLC or a corp.
The LLC would be MUCH easier, because it would be a continuation of the exact same business, with ALL the same tax attributes. I.e. there would be no dissolution of the partnership, meaning no special tax return, etc.
Formation of a corporation would be much more difficult, because it would involve a termination of the partnership. This could have some unforeseen consequences, and should be modeled prior to the formation of a corporation.
Termination and formation of a corporation MIGHT have some very advantageous consequences for tax purposes, but in those cases, there is a risk that some aggressive IRS agent would claim the sole motivation for the transaction was avoidance of income tax. So then it would be best to document the non-tax reasons for the change, in the partnership's minutes.
I think the most common reason for switching from Ptr/LLC to corp. is a preference for the State laws governing corporations, e.g. shareholder rights v. ptr rights. There is often some sort of fundamental reorganization or relocation involved, or estate planning considerations. There are also various Federal laws, other than income tax, that apply differently to corps. and non-corps.