1/ "Digital currencies have a wrinkle that makes them even better in this situation. If you’re still a bitcoin believer, you can use a quirk in the rules to sell and reinvest right away without running afoul of the Internal Revenue Service"
Hey Mike, my name is Brennan and I'm the President of BitTaxer, and we make a software that deals with these types of situations.
TL;DR - Either method accomplishes the same thing, and gets you the desired result.
As it stands currently, coin to coin transfers/transactions are treated as the sale of one coin, with the proceeds going to the purchase of the second coin, so essentially as two transactions.
Using your example, to sell BTC for Tether, it would be a sale of BTC (which triggers a taxable event for capital gains purposes) and then a subsequent purchase of Tether. Using your other example, the sale of BTC for fiat would also trigger a taxable event (the disposal triggers taxable treatment), so either way should help to pare losses I'd imagine at this point.
We recently had an article published that should answer quite a few common questions here - https://smartereum.com/45736/cryptocurrency-taxes-crypto-tax-questions-answered/
Hope this is helpful!