We cannot advise what you should do. But we can indicate some aspects of the relevant law.
The particular license that a developer grants is not as important as who holds the copyright, and the terms of the contract between developer and client, which may include specifying the license.
Let us call the developer D, and the client C. D will hold the initial copyright for the software unless it is a work-made-for-hire (WFH). In the united-states, under 17 USC 101, a work can be a WFH in two ways. Either a) the creator (D) is a regular employee of C, acting within the scope of D's employment, or b)
there is an explicit written contract saying that the work is a WFH, and the work falls into one of a limited set of categories. Neither a) nor b) seems to apply to the situation described in the question.
That the developer (D) grants a license to the client (C) shows that D holds the copyright. A transfer of copyright would be a different agreement (and must be in writing in the US).
C and D will want to have a written contract that specifies who holds the copyright, and what rights each has. This will affect the price for the work.
Since C wants to create a company based on the software to be created, and sell interests in it or acquire funding, D will need to be able to clearly show that the rights to the IP involved are secure. D will want either the transfer of the copyright, or at least a permanent, non-revocable license. D may well want such a license to be exclusive, as well. D will not want any opensource or copyleft license to be used.
Note that even if the agreement transfers the copyright from D to C, under 17 USC 106A D retrains the right to assert authorship, which cannot be transferred waived. Many other countries, particularly European countries, grant a similar "moral right" to the author. This right could be mentioned and acknowledged in any contract between C and D.
If D's primary interest is to be able to show others the significance of D's work, that might be sufficient. If D wants to be able to resell the developed code, or part of it, then a transfer of copyright will not serve D. In that case C and D should agree on just what rights D will have. D should not attempt to hide from C any intention to resell the code, or parts of it, to others later.
