Which Prime?

I can't give you any compare/contrast opinions as it's my only prime, but I use it for my kids (including a infant) and the speed and "noise" is fine. It has yet to cause someone (even the baby) to look at me when I focus if they weren't already.

I definitely wouldn't consider myself an expert in comparing lens IQ. However if you have seen samples and are satisfied with the quality don't let the noise and speed scare you off too much. In real word use of trying to catch a baby doing things quickly and in low light it has captured the moments I wanted both in terms of the moment in time and detail.

So if you're looking for a "value lens" for specific need, I would definitely consider it.