The plastic grip on a steel G3 trigger housing is very similar in feel to a polymer MP5 “Contoured” trigger housing (which is named for its subtle finger swells, arched palm swell, and thumb rests indentations). This hand-filling grip can be too large for individuals with small hands and/or short fingers, making it difficult for them to reach the safety selector.
This is why the “Navy/Ambi” style “straight grip” trigger housings are also popular. The Navy/Ambi grip is more narrow than the Contoured grip, which allows for a shorter reach to the safety selector.
Some folks also prefer the “bullet pictograms” which are stenciled onto the sides of most Ambi trigger housings, rather than the simpler “SEF”, “URJ”, “0 1 30”, etc. markings typically present on Contoured trigger housings, even when used on semi-auto only firearms.
To add one more variant to the mix, there is a Navy trigger housing which is marked SEF, just as is the Contoured trigger housing. Both of those use the same SEF trigger pack and (polymer style) safety selector. Some MP5 Navy SEF trigger housings have been made by modifying former German Bundeswehr G3 SEF polymer trigger housings. It’s a very easy modification, and those trigger housings are relatively cheap, making this a popular choice.