Personally, I haven't found the FA brass bead or fiber optic (which are a smaller diameter) front sights to be a significant improvement for me at dusk. I find a Leupold 2X scope to be better for my use, and a red dot to be still a little better as darkness nears, although the light intensity needs to be turned down as it gets darker.
What works for me though, may not be right for everyone. For example, I've replaced all my FA standard ramp front sights with undercut patridge sights, because the standard FA ramp is just awful for me to see properly.
By the way, as to barrel length, don't forget the 6" FA option. A T'SOB or similar base, and a 2X or even 4X Leupold will work fine with a 6"er.