Is there such a thing as a "standard" interest rate? One of the main factors in determining the rate on a loan is the risk of default. For a newly incorporated company -- that's going to be high.

Both partners might work as hard, but they don't share the same risks. One has put X thousand more into the business, and they should be compensated for that additional investment. The fairest way to do that is to consider what terms you'd find reasonable on an investment of similar risk.