help me decide betwen 2 road bikes silverback strela 2 (new) and gaint defy 3 ( used)

[abdu]

Please help me buy my first road bike, i have found 2 bikes one used and the other new.

Used bike is giant defy 3( 2015) about 600 US, where as the new one is silverback strela 2 also 650 US.

I do await your valuable advice in buying one of this bikes.