The 1/20 AMX is much nicer than the Gremlin. The AMX is full detail, so it has a full engine, folding front seats, detailed chassis and suspension and lots of options for customization. It even comes with racing wheels and slicks for drag racing.
The 1/20 scale is actually pretty decent. It has working steering, engine detail is good and overall looks well proportioned. I think it shows it's age a little in places. The dash and interior could be better detailed (it's still a tub-style interior, knobs are just barely bumps on the dash), the headlights are part of the front ends chrome, etc. But it's definitely worth building.