The Stakes Are High, And All Sides Know It
The most important factor in determining to transfer Bitcoin or USD to Al Salvador is to establish whether Bitcoin has a strong trend of big increases or decreases in value.
If there is no trend and Bitcoin is volatile the most important factor would be in which currency the commitments of the El Salvador resident is denominated in. In this scenario if there are no specific currency denominated commitment and you are a gambler, bet on your preference.
Only when the value of Bitcoin vs the USD is stable, the cost of the transfer would become a determining factor as long as you may easily covert from the one to the other in Al Salvador.
At the end of the day, as I see it, it all depends on the currency of your commitments. If goods, services and loans are priced only in Bitcoin and you earn Bitcoin this experiment might succeed (as long as you are not allowed to buy goods overseas in USD) as there is no exchange risk. If you do not earn Bitcoin, history has proved that you will be on a hiding to nothing, you will also receive less and less Bitcoin for the value of USD transferred from abroad and you will only be saved if goods, services and loans are priced in both Bitcoin and USD. If you earn bitcoin and you can choose in which currency you want to pay, history dictates that you hold onto your Bitcoin as long as possible and only pay in USD.
However history has also therefor proved that if you are an employer, seller, lender or provider of a service and you make your prices in USD and Bitcoin, you will only do business in USD and the Al Salvador Bitcoin experiment will be a failure. God helps the employer who contracts to pay his workers in Bitcoin while prices are quoted in both.
If history is proved wrong, the winners will become losers and the losers will become winners.
The cost of the transactions does not matter at all, it is the currency goods, etc, are priced that will determine how long the experiment will take to fail.