So you are in Australia, are you noticing a difference with the blue tick on S5 compared to M8? I have both too.
Yes, I am in Western Australia, in an area that has 4G coverage (Telstra), but it has poor reception (I even have to suffer with satellite Internet!, but that is another story). I am in a small area that is classified as a 'black spot' (patchy coverage).
On my Nexus 5 and HTC M8, my signal will fluctuate between 0-1 bars, but the phone is usable.
On my iPhone, Samsung Galaxy 4 and Samsung Galaxy 5 I always have one bar, and this can increase to two bars. Once I leave the house and start driving, then my signal will generally increase with all phones. I have active SIM cards in all phones and haven't had any issues with missed calls or delayed messages.
Once I am a few minutes away from home, my signal increases on all phones to similar levels of reception.
I find the iPhone and the S5 have the clearest calls, followed by S4, then M8 and finally Nexus 5.