I don't know much about the post office monthly income scheme. However, I can help you with Mutual Funds Monthly Income Plans.
A typical mutual fund monthly income plan invests approx 70-80% in debt instruments like bonds, company deposits, commerical paper, etc. and remaining 20-30% is invested in equity.
The average rate of return for a mutual fund mip is approx 11-12% p.a. for the last 15 years. However, the return is not guaranteed.
You have an option to receive monthly tax free dividend or let the amount accumulate and compound.
Monthly income plans from mutual funds are taxed as Debt funds. That means, you have to pay 10% tax on returns without indexation or 20% tax with indexation.