Exchanges like Gemini and bitstamp have real dollars. Tethers are just tokens that are offered by a company that is reassuring you they're redeemable for real dollars. No one but that company values them as real dollars.
Read this from here
https://tether.to/legal/ 'There is no contractual right or other right or legal claim against us to redeem or exchange your Tethers for money. We do not guarantee any right of redemption or exchange of Tethers by us for money. There is no guarantee against losses when you buy, trade, sell, or redeem Tethers.'
Do you find that reassuring? Gemini has fdic insurance for your dollars, a banking charter and actual regulation. There is no comparison.
Yes, even if you left dollars on an exchange the sale of your bitcoin would be taxable. I'm willing to guess that the same applies for tether if any regulator cracked down on it.
An alternative is real dollars on btc-e which doesn't care who you are, but that's a risk too.
