the difference with bitcoin is obvious! it is the centralization of Tether. it is owned by Bitfinex parent company and is 100% controlled by them. and that means it is highly risky to use it let alone to store it for long term.
on top of all that, this company has always been shady about Tether. they assure you that it is backed by dollar with 1:1 ration but they have never been able to provide any proof or guarantee nor were they ever properly audited. they always evade it!
They have rights to issues as many Tether USD as they want (if we assume that they don't really have dollars back for Tether USD at the ratio 1:1 as they claimed to do). There are many FUDs on this (but I thing they are rights) and manipulated the market with bad news relate to Tether USD and Bitfinex times to times. No one knows the severity of this issue, but I believe there are issues with Tether USD.
as for market cap, that is the problem nowadays with people, they think just because a website called coinmarketcap.com ranks a coin on top then it must be good or safe to use! they should really stop doing that and start looking into altcoins from perspectives that matter like the things i mentioned above. market cap is meaningless.
In fast changing market like crypto, market cap does not make any sense. Price can skyrocket and lift total market cap up and in contrast, when price flash dumps we will see marketcap drops quickly.
There is another thing, premined, instamined coins/ tokens and hold by their teams. Those amount of coins/ tokens should be excludd from marketcap.