A quick glance at the script (I may have missed important parts) shows that everyone costs the same so you should only need to consider the effectiveness of the type of hire; every type has a manual click button effect and a daily automatic effect that scales with count which affect growth and product quality which affect users and valuation.
And quality caps out at 4. So I guess focus on hiring developers/marketers and writing code until product quality is 4, then just marketers until you run out of money. Poor designers...
=
Developer:
write code: growth + 0.3, quality + 0.2
automatic: (growth + 0.3, quality + 0.1) * developers
=
Designer:
create design: growth + 0.1, quality + 0.2
automatic: (growth + 0.2, quality + 0.1) * designers
=
Marketer
promote product: growth + 0.5
automatic: growth + 0.5 * marketers
=
And quality caps out at 4. So I guess focus on hiring developers/marketers and writing code until product quality is 4, then just marketers until you run out of money. Poor designers...