Which base currency should I select for the loans I want to invest in?The question which base currency you should select for your investments depends on two things: the loan purpose and your view on the future bitcoin price.
In case of bitcoin mining loans, bitcoin as a base currency typically is the better option. Miners and people who are buying mining contracts have a more or less foreseeable income stream in bitcoins. Therefore they typically select bitcoin as a base currency for their loan. If you as a lender are also looking at your bitcoin holdings regardless of what is happening to the bitcoin price, bitcoin base currency loans are for you.
If you believe that bitcoin price volatility continues to be higher than that of fiat currencies, you should think about the US dollar as a base currency. Note that for these exchange rate pegged loans all payments are still conducted in bitcoin only.