There's been a lot of doubts regarding Tether and whether it is really backed 1 to 1 by US dollars.
Some Twitter accounts have been sounding the alarm for some time now, in particular an account called Bitfinexed:
https://twitter.com/Bitfinexed
This caught the attention of Tone Vays and a few other popular Bitcoin personalities who have been trying to get to the bottom of the worrying allegations made by Bitfinexed:
https://twitter.com/ToneVays
Tether and Bitfinex have been playing down the allegations for weeks. Tonight however Tether made an official announcement conceding there is indeed a problem and there's 30 million missing/hacked!!!!
https://twitter.com/Tether_to/status/932812123020787714
- Tether_to
This proves once again that trusting a centralized entity or gateways to really backup their assets is wishful thinking unless regular audits and tight regulators frameworks are in place.
What is Tether backed up by at this point we don't know. We saw that its value so far was only based on a lot of misplaced trust.
However we know that the SBD (Steem backed Dollar) is backed by Steem and it is all transparent on the blockchain! No need to "trust".
As a trader right now I would feel safer parking my savings in SBD rather than Tether.
What makes SBD a problematic alternative to Tether though is its difficulty in maintaining the US dollar peg. A solution by 
https://steemit.com/sbd/@timcliff/saving-sbd-proposal-to-restore-the-usd1-usd-usd1-sbd-peg
Either Steem delivers a more dependable US dollar peg with its SBD or calls to get rid of it will remain:
https://steemit.com/ned/@krnel/ned-on-sbd-does-the-community-want-to-continue-to-be-paid-out-in-steem-dollars
With Tether and other centralized pegged solutions failing I still see the SBD as a useful asset for the time being. But with a better implementation it would really shine and bring a lot more value to Steem.