[Brightcolours]

If you need to decide between f/1.4 or f/1.8, I'd say the aperture alone should not be the decision to make, but mostly it's a useless discussion. Wide open lenses need more accuracy for focusing, but also deliver more light for the focusing unit. Even if you don't take pictures with a wide open lens, you benefit for the more simple definition of sharp areas.


And in turns of DoF, yet another useless discussion. DoF gives you no room of sharpness, it's room of increasing less blur, but maximum sharpness exists still only in a shallow area.


If you have a wide open lens, you can use more bokeh, if you have no wide open lens, forget nicely blurred background. That doesn't mean you don't get great portraits.

It is a misconception that for PD AF, bigger apertures "deliver more light to the focussing unit".  If your PD AF point is a f5.6 point. an f4 lens gives the same amount of light to it as a f1,.4 lens.  Think of the AF point as having its very own aperture, only seeing that very central part of the lens. If it is an extra sensitive f2.8 AF point, f2.8 gives it the same amount of light as f1.2.