V notch vs Square notch rear sight?

I am ready to put my order in for either the TRR8 or the M&P R8 and up until this point I have been leaning toward the M&P R8 as I prefer the built in front rail just for maintenance and strength sake. Then I looked at the sights. I don’t think I am a big fan of the V notch rear sights on the M&P R8. I think the square notch of the TRR8 would be easier to quickly acquire your target. I am fairly new at this so I am asking you:

Do you prefer V notch or Square notch rear sights or does it not really make a difference?

Are these rear sights something that is easily interchangeable?