A fractional CTO and a part time CTO both offer their technical expertise to companies without requiring the hiring company to add the full time cost of a very senior hire to their payroll.
Either approach can allow a company to access a level of expertise they may not be able to afford on a full time basis.
However, there is a subtle difference between the two engagement models. Typically a part time CTO would be engaged on a part time basis by a single company.
A fractional CTO would tend to be engaged by a portfolio of companies, each one purchasing a fraction of the CTO's time.
Deciding which type of CTO is optimal for your business will depend on several factors and it is worth keeping in mind that there is plenty of overlap between each model, at times with little discernible difference to the hiring company.
A fractional CTO will be gaining experience based on the challenges faced across their portfolio of clients. This has the potential for a fractional CTO to gain exposure to a more diverse range of technologies, processes, tools and suppliers.
However, a part time CTO might offer more focus and greater availability for the single client they serve.
A key feature of both the fractional CTO and the part time CTO is that they do not assume the engagement is short term. Unlike an interim CTO it is quite normal for a fractional CTO to stay engaged from start up through scale up and up until the point where a full time CTO is required, who they are likely to assist in hiring.