Not so clear cut. A needs to put something down for the house, let's say the standard 20%. B invests those $20k at a conservative rate of return, say 5%. After

Not so clear cut. A needs to put something down for the house, let's say the standard 20%. B invests those $20k at a conservative rate of return, say 5%. After 15 years, B has $41579. Also, A will have to pay maintenance costs on the house, which are very hard to estimate. On the other hand, A's house might appreciate, he gets a tax deduction on the interest paid and B's rent might go up over time. My point is there are a lot more factors at play here.