I play both tennis and squash at a pretty high junior level.
Tennis has more variation eg. spin, power and tactics. Tennis has all the attributes needed for other racquet sports so it makes it easier for a tennis player to adapt to lets say squash. The squash technique is less technical than a tennis shot. You can't play tennis with a squash grip
But you need to be way, way fitter to play squash than tennis at the highest level.