I took a closer look at the available loans in Cities Skylines. They are:
Silver Sunset Bank
Amount: 20,000
Payment Plan: 52 Weeks
Interest: 5 %
Weekly Cost: 403.85
Total 21,000 c
Global Credit Inc.
Amount: 60,000
Payment Plan: 260 Weeks
Interest: 10 %
Weekly Cost: 253.85
Total 66,000 c
Pyramid Capital
Amount: 200,000
Payment Plan: 520 Weeks
Interest: 15 %
Weekly Cost: 442.31
Total 230,000 c
These loans appear to be
annuity loans (the payment plan has a fixed periodical cost). The interest is calculated as (Total / Amount) - 1, so I converted it to the equivalent annual interest, and got the following:
Silver Sunset Bank: 9.66 % per year
Global Credit Inc.: 3.86 % per year
Pyramid Capital: 2.86 % per year
The equation is: A = S*p/(1-(p+1)^(-n)), where A is the weekly cost, S is the amount, p is the periodical interest (in this case interest per week) and n is the payment plan. Using the
bisection method it's then possible to solve for p. Finally, p*52 gives the annual interest.
So although the game makes it appear as if Silver Sunset Bank has the best deal (5% interest), the annual interest suggests that it's actually Pyramid Capital's loan that is the best. At least if you intend to invest the money to generate growth. If it's just for luxury consumption (e.g. building yet another fancy monument) then Global Credit Inc. is probably the best as it has the lowest weekly cost.