Both are stunning as adults if you can find good ones. The blue/greens in both are beautiful and unique as adults. I've found the GT more owner responsive whereas adult dempseys tend to just swim back and forth like arowanas.
is it a species tank or a comunity i'd go for GT with species and JD for community
if it is solitary i would go for a more colorful and interactive species of cichlid may i suggest a parachromis species!! just don't get dovii unless you have a big tnak