The argument would be that after 30 years of 2.3%/yr inflation, the 300k house would be worth 600k in future dollars, and the 100k house would be worth 200k.

However as 15% interest rates implies an inflation rate far higher than 2.3% that argument breaks. With 7% inflation on a 100k house it's worth 760k after 30 years, more than the 600k in the 300k range.

Of course what that means in terms of big macs you can buy is different.

Not only that but if your salary keeps up with inflation, and you overpay the mortgage, you'll have cleared the 100k/7% inflation mortgage in 10 years but it will take 18 years to clear the 300k mortgage.

Sure, having your wage not decrease in real terms is a different challenge, but if that does happen then you're far better off with the 15% interest rate situation.