We have to do a lot of oversimplification just to come up with a rule of thumb. If we assume all the following to be true:
- the house will not be your primary residence so you can't take a tax deduction for the mortgage interest
- won't be used as a rental
- inflation will be the same in the US as Russia
- ignore currency exchange rate fluctuations
- if you invested more in the home and had lower mortgage payments, you would re-invest the difference back in the IRA or another productive investment
then any amount put toward the house in excess of the minimum down payment gives you a return on investment equal to the interest rate (5-6%). Do you think you will get a larger return in your IRA? It depends on the economy and your level of risk tolerance (aggressive vs conservative.)
Changing the assumptions affects the result. If you use the home as a rental, you can deduct the mortgage interest as a rental expense, so that leans toward a larger mortgage. If you expect inflation to be higher in Russia, that makes future money cheaper (a $1000 mortgage payment 10 years from now is less real money than $1000 now) so that also argues toward a larger mortgage. (If inflation is the same in both countries the effect is a wash.) If you don't have the discipline to reinvest the mortgage you won't pay, that also argues for leaving the Roth account alone as much as possible.
Also remember that if you buy the house for cash, you have just changed what is presumably a diversified and liquid portfolio into one that is heavily invested (50%) in a single, illiquid asset. So there is some risk involved in doing that -- is the house in a good location, will it appreciate or at least hold its value, is the Russian economy and government stable, or is there any risk you could lose the house in a government action or the house will lose value due to being in an unpopular or dangerous neighborhood.
