The ts450 is an old unit replaced by the core & pro series.
Don't confuse it with the 550 & 650 because its not even the same platform.
Group regulated 406w 12 v rail (at a push its a 430w unit at best) , 3 year manufacturers warranty & a single 6+2 pin PCI express connector.
Yes its a seasonic so its still very good quality.
The corsair is a modern dc-dc unit , twin 6+2 pin PCI express connectors, full 450w available on the 12v (actually tested to output 520w continous loads) , 5 year manufacturers warranty.
Ignore the shop warranty , your warranty is with corsair , unless you live in a third world country any RMA would be done directly with them .
Do you honestly trust your shop 4 years down the line to stand by their warranty if your PSU packs up ??
Because I wouldn't & I live in the uk where consumer law is incredibly stringent.
I have a feeling they are trying to offload their old stock on you.