You should chose rules whose character generation sequence creates the kinds of characters you want in your game. It's your game and, as GM, you should consider these factors in advance. Choosing the Setting Almost as important is the choice of setting. In some cases, the choice of rules will dictate that. The rules choice has great bearing on setting choice as well.
Some rules naturally incline one toward particular kinds of settings.
As always, though, do what you find most comfortable. I have made some odd rules choices in my day and made them work well. The choice remains yours. These two categories are necessarily fast and loose. Some SF genres, like cyberpunk, could easily fit into either category.