According to (IIRC) the film's science adviser, the intent of that scene is that they are still decelerating relative to the station on the chute they are snagged on, and Clooney's charscter believes that the line doesn't hace sufficient strength and may snap with both of them but will hold with just Bullock's character.

If that's not just a post hoc rationalization, the film certainly doesn't seem to convey that effectively, though.