Guzhengs seem much heavier and bulkier in person as compared to the portable Guqins.
You will also realise that Guzhengs have much more strings than Guqins (21 and 7 strings respectively). Hence, the Guzheng is more capable of producing a wider range of notes than the Guqin.
The Guzheng also sounds louder and resonant that the Guqin, due to the size difference. As the Guqin is lower in pitch, the volume of the Guqin is not as loud and bright. Thus, the Guqin will sound mellower while bringing a sense of tranquility to its timbre.
Techniques in the Guqin are said to focus on the left hand, whereas the right for Guzheng.
Guqins only require a table to be played on, whereas the regular sized Guzhengs require a set of stands, as well as a set of fake nails to be worn in order to be played.