Bitcoin is the ancient crypto currency and it has been around for over a decade. It was created and that time its cost per coin was $0.02 and now it costs more than $9000. The majority now wont be able to access a full bitcoin, they can buy fractions of it but you know it frustrates to only a small fraction
Birth of other Crypto coins
After bitcoin hit the ground then a lot of coins started to emerge on the planet and based on blockchain. USDT came under the company called Tether LTD and the coin is said to be backed by US dollar.

The main difference between these two:
Coin supply - USDT has larger supply of 111Billion coins as compared to Bitcoin with 18Million
Not volatile - Bitcoin is very volatile and one can make or loose money in the process. Its price is subject to change at any different given time but USDT is not volatile, it maintains its value of $1 per coin though it changes hear and there
Stability - USDT is very stable as it maintains its value constantly as compared with bitcoin which goes up and down
24Hr Capitalization - USDT has large daily capitalization of $15B AND Bitcoin has $45B which shows USDT is more liquid as compared to Bitcoin
Therefore we can say Bitcoin is a coin for investment as people can make money when it goes up and down but as for USDT it is an assets and can be a safe haven which maintain its value over time and if you want to make money on this coin you wont make any instead go for bitcoin as it is pumped and dumped