@Account Closed For rental property your tax shelter is depreciation and 1031 exchanges. Theres not much in the way of tax sheltering notes besides putting them in IRAs. This is the biggest reason I can see why notes are often put into IRAs
When comparing long term opportunity cost between rentals and notes, the big difference besides depreciation, is that rental property should appreciate and rents should increase over time. In this way you could argue that rental property is inflation protected. A performing note does not appreciate, in fact that fixed monthly payment will be less meaningful over time due to inflation. See inflation-induced debt destruction at the bottom of this article, you would be on the other end of that stick as a note owner.
I think you could also argue that leveraging is easier/more practical with rental property, which might be the biggest factor in long term growth.
Thats not so say one is better than the other for long term growth, just that there are different things to consider. You should be able to get the same amount of growth going either route.