I ask this because I am getting ready to buy a used USP .45 for $650.
What is the difference between the two variants and how can I tell?
Which variant is better and why?

variant 1 has a safety/ decocker on the left side-----good for right handers

variant 2 has a safety/ decocker on the right side-----good for left handers

otherwise, both are mechanically the same.

Quoted: I ask this because I am getting ready to buy a used USP .45 for $650. What is the difference between the two variants and how can I tell? Which variant is better and why?

Also remember that there are 9 variants for USPs.

Variant 1 DA/SA with control lever (safety/decocking lever) on left
Variant 2 DA/SA with control lever (safety/decocking lever) on right
Variant 3 DA/SA with control lever (decocking lever) on left
Variant 4 DA/SA with control lever (decocking lever) on right
Variant 5 DAO with control lever (safety lever) on left
Variant 6 DAO with control lever (safety lever) on right
Variant 7 DAO with no control lever
Variant 9 DA/SA with control lever (safety lever) on left
Variant 10 DA/SA with control lever (safety lever) on right


And on top of that, there's the old DAO trigger system and the new LEM DAO.

FYI, there is also a V8 but it is the same as the V7 except it has very powerful night sights, too powerful in fact because the US will not allow them to be imported.
Something about sterility when tucked inside the front waist band  (kidding).